Travel agent questions

I’ve never used a travel agent, but as I’ve gotten busier with work it’s just meant that I travel less because I have less time to plan! I’ve been thinking more lately about using an agent.

So a few questions:
1. Do they book *everything*?
2. It seems like most don’t get paid by the client and get paid by the places you stay? Does that mean they have to have a relationship/deal with the hotel or airline? Will the place we stay and flights then cost more??
3. Can they book me in a place that has a kitchen or only hotels?
4. If I have food allergies, is that something they’d be able to help with (like restaurants that can accommodate the allergies)

Is there anything else I should know???