I think that at least southern Italy made geostrategic sense. It would protect the Balkans from as western assault, which, as we saw with the Normans in the 1040s-1080s, was vital; With southern Italy they were annoying, but without it they were an existential threat all of the sudden.
now to respond to your excellent point of view on the geopolitical importance of the imperial control of Southern Italy, as well as the search for important allies on site ( which Manuele initially had, only to unfortunately find himself having to support the rivals of his old ally ), at least for defensive purposes, I am about to quote myself from another discussion, with a fairly long comment
To understand why Manuel's Italian campaign went badly, we need to go back to the second crusade, when we find Manuel who is monitoring what happens with the crusade, he receives information that said that the fleet of King Roger II of Sicily had set in motion, the Sicilian fleet was commanded by George of Antioch ( a Rhomanoi who served first under the Arabs and then with Roger II, becoming his Amirus Amiratus, i.e. today's equivalent of the rank of prime minister and commander-in-chief of the army ) In a very short time the Sicilians conquered Corfu, and raided Athens and Corinth, reaching as far as Thebes ( the nerve center of the empire's textile industry )
Once George of Antioch was paid almost the equivalent of a triumph in Palermo upon his return, as a result of this Manuel turned to the Venetians in search of allies, and in 1148, Venice promised to lend aid to Byzantium with the his navy for six months, in exchange for more commercial privileges, in April of the same year Manuel was ready for the punitive expedition against the Normans, but the situation on the northern border suddenly worsened : when the Cumans entered Byzantine territory and the Venetian ships were stopped due to the death of the doge and a violent storm that had broken most of the fleet's oars. Only in late autumn did the navies of the two states manage to carry out the maritime blockade of Corfu
In the same period Manuel had to go alone to Thessalonica, to reunite with Conrad III of Swabia ( who in the meantime fell ill in Ephesus ), the emperor then decided to take him back to Constantinople, where he had him treated ( in some stories it is said that it was Manuel himself who took care of Conrad ) meanwhile the crusade was going badly, and many blamed this on Manuel ( among them there was also Louis VII ) he returned from Palestine, Conrad was invited by Manuel to Constantinople, ( a friendship had been created between the two ) where at Christmas 1148 a marriage was celebrated between Theodora ( Manuel's niece ) and Duke Henry II of Austria ( Conrad's brother ) furthermore they agreed to carry out a joint campaign in Italy in the shortest time possible
before actually starting the campaign, Manuel had to face the Serbian revolt ( financed by Ruggero ) as well as a fleet of forty Norman ships ( again led by George of Antioch ) which, once they arrived under the walls of Constantinople, had sacked numerous patrician villas along the coast of Bosphorus, launching several arrows as a provocation into the area of the Great Imperial Palace. Furthermore, to avoid the start of the campaign against him, Roger supported the Bavarians ( who were in revolt against Conrad ) to prevent him from undertaking the campaign in Italy and joining the Rhomanois army
but when everything seemed ready for the start of Manuel's Italian expedition ( who had earned the support of Venice, Pope Eugene III and naturally Conrad ) the death of Houestauffen unfortunately upset the plans ( since Frederick I known as Barbarossa , could not fulfill his uncle's promises, due to urgent problems in Germany ) when in 1155 the news reached Manuel that the barons of Puglia ( who had never looked favorably on the Altavillas ) were intending to rebel and were asking for his support, he did not lose heart and sent his two best generals to Italy : Michael Paleologos and john Doukas, with the mission of contacting the Apulian barons and possibly if Frederick was still in Italy ( he was crowned emperor just the year before, and in the following months he was trying to impose his government on the cities municipal councils of northern Italy ) with the aim of meeting him and asking him if he would support the Byzantine Empire against the kingdom of Sicily ( a meeting which took place in Ancona, but which was opposed by the German barons who did not want to prolong their stay on the peninsula any longer ) for the first year the campaign proved to be a resounding success, given that the Byzantines managed to recover the whole of today's Puglia, furthermore even Pope Hadrian IV himself was contributing ( which from a diplomatic point of view was a real coup of prestige and great skill on the part of Manuel ) but unfortunately the death of Michael Paleologos and the flight of the Apulian barons forced the remaining army to fight King William in clear inferiority, and they were obviously defeated
therefore there were many causes that led the Byzantines to the failure of this campaign, but if it had occurred a few years earlier, allowing for greater collaboration between HRE and Byzantium, then it is probable that the Altavillas would not have fared so fortunately like Otl ( it could certainly happen that due to problems between the allies, the expedition fails or is only half successful, but at least in Constantinople for a few decades they wouldn't have to worry about a possible attack from the western front of the Empire ), it is also likely that this campaign ( if it is successful ) unintentionally strengthened the Houffenstaufen's position in the Empire and in Italy, perhaps allowing Conrad his coronation as full-fledged Emperor, particularly if it is actually begun as scheduled in 1148/9, finally, a further marriage in the future between the two imperial families cannot be ruled out, in particular there was talk of marrying Henry Berengar to a Porphyrogenite princess