I use it whenever I need to debug a piece of code. For example I like to play with fuzzers, so when one of my fuzzers finds a crash in whatever software I'm fuzzing I'll re-run it in a x64dbg and see what's happening.

I'll also use it to debug my own assembly code if I'm working on a low-level project. Projects include trainers for offline games, low-level system inspection tools and/or security tools (such as monitoring which process opens which files etc).

Sometimes I need to reverse malware for my job, I'll use both IDA (for static analysis/keeping notes) and x96dbg for dynamic analysis.