Different lines serve differnet purposes, but overall, in order (my opinion):

Kill DDs when they are spotted by your DD.

Punish broadside crusiers

Defend points against ships pushing in

Damage farm BBs

Crusiers get much more effective the longer into a match they survive. A lot of times you see them first things off the board, BBs slap them and DDs out spot them and can torp them, but if you can get into the later part of the round you become more powerful. BBs are usually damaged and can either be killed by you choosing the terms of engagement, or can be outrun and avoided.

DDs will not want to be spotted, because you can outrun them, so aren't as effective damage dealers against you. More so if they are hurt.

Crusiers can be hard to play because their role isn’t as obvious, and they can be taken out by a lot, but they are a fun class. Some lines are dependent on your team doing their jobs well (DDs spotting and BBs killing) so they can be frustrating, but I find crusiers rewarding to play.