Why are processes needed in Unix?

Question

*nix-like systems

I gave a lecture on processes in Unix OS, they "told" about identification, process hierarchy, contexts, process states, that in UNIX OS a new process can be spawned in the only way - using the fork () system call. Perhaps too noob question, but why do they need processes at all? Why call the same fork () so that a child process also appears?
P.s. considering only uniprocessor systems

Answer 1

In computing, a process is an instance of a computer program that is being executed.

After all, you can have a browser, messenger, text editor running at the same time. They do not know anything about each other, but they work at the same time. These are different processes.

Answer 2

Question level answers and questions mail.ru. Either read the lecture again, or read a normal one, in which it is described what kind of "processes" they are.
And only after understanding, go to forks, priorities, etc.