When you want a good steak, you don't go to a fast food restaurant. A good steak can be found at a good steakhouse.
If you need an painter, you hire a painter, you don't hire an electrician who can paint.
Many contractors specialize in just one thing. Specializing in just one area of expertise means they are most likely very good at what they do.
Many other contractors offer a variaty of things that they claim to excel at; and they may - or may not - be good at what they do.
Having a "one stop shop" sounds convienent.
If it's a huge company, their claim might be true, otherwise; they could be just trying to make additional money in a field of work that they really have no business promoting, and quite often; they will sub-contract what they really don't do.
This is not always a bad thing, but the client or homeowner needs to know this upfront, so just ask them before signing that agreement.