Travel agents come in all sorts of shapes and sizes and they are by no means all equal. Agency owners and managers have taken advantage of computerisation to dumb down the staff they employ. The same is true of airlines; about a year ago I enquired of an airline what the cheapest fare was from A-B. Reservations couldn't tell me (previously they had always been able to). They could tell me the cheapest available fare if I gave them dates but THE cheapest fare on the route had to be dealt with by another group where the experts were.
A good agency will walk you through your plan and question why you want to do what you want to do so that they can suggest alternatives and tell you why they think the alternative may be better for you. Sometimes the alternative will be more expensive, other times they may be able to save you money because they have that intangible called expertise, other times the cost may be exactly the same but the routing might save time or allow more connection time or .......
A good agent will then be monitoring your reservation and ticketing to ensure that everything goes to plan and to help you should an unplanned change occur. But help isn't necessarily free.