Legitimately, a business could do anything.

A plumber could do new construction, flood cleanup, repair and maintenance. They could also expand into HVAC, do garage doors, or anything where someone runs by a house to do work.

The purpose of the mission statement is to help everyone in the organization understand the things that the company is striving to be good at and boundaries of what they're going to work on.

So, a mission statement would help a plumbing business avoid the mistake of having a junior plumber develop a plumbing app for do-it-yourselfers, or avoid having plumbers carrying around carpet cleaning equipment because those services are outside the scope of what the business wants to be good at.