Spatial planning is a critical first step in any interior design and should be done at a very early stage in a project. It involves working out how a room works aesthetically and ergonomically, where focal points are, and going through some real practical details. It should both be informed by and inform your architecture, and should be the bedrock on which other critical and early decisions (eg; power, plumbing, and fine tuning door locations) are based.
Later on in the project it will contribute to decisions around things like the size of particular items of furniture, and the fine detail of the design. The big watch out though is to do it early enough, and if you are at all unsure bring in an expert. Any future interior related decisions need a firm bedrock on which to build.