There certainly are reasons (not a value judgement), some of them cynical:

- Crimea wants to control the canal from the Djnepr, which had been shut off by Ukraine.

- Russia wants the coast line to control more gas fields in the Black Sea. The U.S. of course also want to control them by proxy (see Burisma/Biden). Snake island also controls some of the fields.

- Rightly or wrongly, Russia is scared of NATO expansion.

I think it may be the case that the great Russian empire theory is a pretext that is put forward by Putin for his own population and eagerly taken up by the West for propaganda. Attacks on Kyiv (stalled) and Kharkiv could be diversions from the south. As noted above, the Kharkiv attack does not make sense though.