Because you don't want every process to have control over the file system and other processes. This limits the impact of a misconfigured service, a compromised

Because you don't want every process to have control over the file system and other processes. This limits the impact of a misconfigured service, a compromised service, or just a plain malicious service... or users of such services. Then, it doesn't necessarily have to be malicious, you can also harm your system by accident. I have done it many times.