The character it gives vocals means many professional vocalists and producers prefer it over much more expensive mikes. At less than £100 they are priced more like a Squier of microphones than a Fender USA...
The SM58 is competent or even good in most other roles if not exceptional, and tough, reliable and repairable, so it's a good choice as a general purpose mike that you can swap in and out without worry. That said, I won't argue that there are other mikes that are better for specific roles, like miking up acoustic instruments.