BB backlines are great because of high HP and they can also shoot at the suicide boats with some LB's, not to mention you can time their main shots to also take out incoming boats. That takes a bit of practice to get down though.

Maybe you can use auto to do it for you, but I hate having my torps wasted so I learned how to do it manually.

Frontline, I run with Bel/Edin/Leander mainly for decent HP + torp spam (time torps smartly to maximize results), and Leander's barrage skill helps too (wide AoE spray).

Haven't ranked high enough for the Lv70 one yet, but the Lv50 one is pretty easy for me running BB main CL escort.

One thing that might help you out if you're having trouble learning positioning is getting the Beaver Badge that boosts your fleet movespeed by +20%, if you think you might need it.

TL;DR: BB backline is good because high HP + they can shoot back. Frontline you want some mobility + spammy damage to quickly erase most boats, torps are great for this.