- market the property on places like rightmove, findaproperty etc
- show prospective tenants around (so you don't have to take time off work)
- take pictures, measurements and construct floor plans
- do credit, CRB and reference checks
- provide a boilerplate tenancy agreement (which you can amend as you like)
- sort out posting of deposits to the Tenancy Deposit Scheme
- collect the rent
The amount of time and hassle all this saves is a lot... Of course there are crummy agents so do your research.
Incidently - your "no risk" claim is a worrying one and suggests you don't know much about the lettings business.