No, in many cases one needs to set them interactively.
Consider for instance something as simple a implementing a shell. Such a program needs to be able to set the environment based on user interaction and this change needs to show up in /proc/$pid/env.
Because the specification of the POSIX shell says that `export` changes the current environment of the running process, not just of any newly started processes.
This is useful to recognize various processes I suppose. I have written code that scans the environment of processes to find particular processes and group them together.
Consider for instance something as simple a implementing a shell. Such a program needs to be able to set the environment based on user interaction and this change needs to show up in /proc/$pid/env.