The main reason usually given is for defense. The Emperor Gallienus first moved the capital to Milan during the 3rd Century Crisis in order to put himself (and his newly created mobile field army) closer to the frontier on the Rhine and Danube. Rome was far away from the frontier while Milan was close enough for the Emperor to react quickly to invasions while far enough away to be safe.

Ravenna is kind of similar, Honorius, an Emperor in the 5th Century CE moved the capital to Ravenna at a time when the empire was in steep decline. Ravenna was seen as a better place for the imperial capital than either Rome or Milan because it was surrounded by swamps which Honorius felt made it more defensible. Ravenna was also home to the imperial fleet and was only a short hop across the Adriatic to the Eastern Empire so offered an escape route should it be put under siege. In hindsight it does seem like a bad move to have relocated the capital from Milan to Ravenna though because Ravenna did fall to Germanic invaders, despite the swamps, and it was hard to project power from the city. Alaric simply bi-passed it on his way to Rome.

So defense was the main reason, Rome had been fine while Rome had pretty much impermeable frontiers and the Emperors didn't have to ride out constantly to meet Germanic threats. But as the pax romana broke down, it make more important to have a capital closer to the frothiness.