Short answer: What a rental agent adds when renting a condo or house in Jomtien, and when it's worth using one.
You can rent directly from an owner in Jomtien, and plenty of people do. Using an agent isn't a legal requirement, but it does solve a few practical problems, especially if you're not already in Thailand or don't speak Thai.
An agent can show you multiple units in one trip instead of chasing separate owners, check the building and unit condition before you commit, handle the lease paperwork in both English and Thai, and act as a point of contact if something goes wrong during the tenancy.
There's no extra cost to the tenant in most cases -- our fee is typically covered by the landlord -- so using an agent generally costs you nothing while saving you time and reducing the chance of renting somewhere that doesn't match what was advertised.