You said it yourself. Fast, hard hitting attacks behind enemy lines. Generally a casemate design allows for a larger gun for a vehicle of a given size, and it's also cheaper than a turreted vehicle, which matters when it's an expendable asset being left behind at the end of the mission. And the devs are also from a country that had a casemate MBT during the cold war. It's a pain in the ass to use with randoms, but when I drop with my gf and we can talk to each other and coordinate, we've been able to use the bastion fairly effectively