You are under no obligation to hire a letting agent. Whether you do will again depend on the nature of your investment. You might just want to use a letting agent to find and vet your tenants for you. If you are in buy to let for the long haul, your property is half an hour away and you have a full time job, you might hire a letting agent to carry out a full management service. Here, as well as finding the tenants, they will collect the rent, prepare inventories, deal with tenant enquiries and sort out repairs and maintenance. For a service such as this they will take between 15% and 17% of the rent you receive.
Letting agents worth their salt should tell you who is renting what in their patch so will be able to help you target your investment at the right tenants. It might be commuters living in two-bedroom flats near the station, for example, or young families in terraces near the schools. However, be warned that most letting agents overstate rental returns available so take them with a pinch of salt. What you're really trying to find out is what is easy to let. And letting agents are not going to recommend something that will be a pig to let otherwise they won't earn any commission from it.
Many investors find letting agents an invaluable tool. But they might not be as useful if you want to target an investment toward student and DSS accommodation. Tenants on letting agents' books tend to be other types, such as young professionals, commuters and unlucky house movers caught between buying and selling. For student and DSS accommodation, local newspapers and the university itself might be the best place to net your tenants.