What are you paying an interior designer to do?
Use their knowledge to design a room that functions at its best by Space planning
Make sure you furniture proportions are best for your space
Install and assemble furniture
Hire other professionals
Project management- they should have all the answers or know where to go to get them for you
Translator between you (the client) and the contractor or trades person.
Design your space to your liking by sourcing furniture that you like
Purchasing furniture while staying under budget