This might seem like an obvious topic, but Interior Designers need Architects just as much as Architects need Interior Designers. Architects and Designers should function like two halves of one brain. The Architect plans out the structure of the building, consulting other professionals (Electrical Engineers, Structural Engineers, Plumbing Engineers, etc.) to make sure their designs will be structurally sound, while Designers make sure the space will function properly. Together Architects and Designer's can accomplish great things, but apart, they can often times over look certain areas outside of their expertise. This is why it is best to hire an Architect to create your structure and Construction Documents to build from, but also to consult a Designer to make sure the layout will function just as well inside as it will outside.
If you have never worked with an Architect or Designer before, think of the Architect as the person who is responsible for building the exterior of your home, making sure it will stand up and remain standing, and making sure your HVAC (Heating/Cooling), Plumbing, and Electricity are placed properly and will work their best. They are also responsible for making sure your home meets all required codes, and that the energy your home will use will have a minimal impact on your community (and your wallet). Designers are responsible for making sure the inside of your home will function to the best of its ability. For instance, you don't want your kitchen placed too far away from your garage because when you bring groceries home, you don't want to be dragging your grocery bags across your entire house or upstairs, etc. You also don't want to walk into a room that has all the light switches placed at the opposite side of the room.
So when you are starting your next big home project, be sure to consult both an Architect and a Designer to make sure your home is the best it can be.