Manual memory management is important if GC will blow your latency, throughput, or jitter budget. Keep in mind that latency can easily be amplified by artificially increasing contention or any number of other issues.

Even something as simple as a constraint solver can easily find that memory management overhead dominates if they don't do something interesting. Work per deallocation can be extremely low in that domain, and anything like checkpointing can make the new memory access patterns sufficiently non-trivial that the GC isn't likely to be able to recognize it.

A lot of software probably doesn't care though I don't think.