Compilers generally instantiate one process per input file. (.cpp or .c files, but not .h) Because the compiler writer can generally be assured that the process is very short lived, compilers are typically written to never use free(). When the object file is created, the process exits, and the memory is freed by the OS. This is generally a significant speedup.

Unit tests. When a unit test crashes, that crash needs to be logged, and it can't interrupt the other tests. The good way to do this is by creating a new process per unit test.

Some web servers in certain configurations will start one process per connection. If you have 1000 simultaneous connections, you have 1000 processes. https://httpd.apache.org/docs/2.4/mod/mpm_winnt.html

That's 3 examples. There are others.