Some perform tasks, but they largely just take up space, and that’s a good thing. Your body’s full of materials that malignantly pathogenic bacteria would love to get their hands on. Bacteria that are largely incapable of doing anything to us take up space that would otherwise be occupied. You’d likely prefer an old squatter living in your walls, rather than a crackhead. You’d probably prefer an electrician, but that’s life. Better they’re largely benign than overtly and desperately malicious.

You factually do have a viral balance in your internal ecosystem. Bacteriophages cull populations, and some viruses hyper-specialize in attacking cancer. There are more examples, but I can’t immediately recall