Getting inside an integrated air defense network to take out strategic targets. It's got what is believed to be a better stealth profile than the f-22 or f-35, while having payload comparable to a b-52. The MQ-9 wouldn't be able to get anywhere close to an air defense network.
Large scale strikes also, but after air defenses have been taken down, or suppressed by SEAD escorts. Keep in mind we only have 21 B-2s and they are very expensive to maintain, so it makes sense to use the B-52 whenever possible.
