Membrane rafts play an important role in the localisation of membrane proteins, allowing different regions of membrane to be formed (which allows control of protein dimerisation, amongst other processes).
By forming more rigid sections of membrane, through phase separation, it is possible to segregate the membrane into sections. These can then be clustered, allowing membrane proteins to be localised. Rafts have a higher proportion of cholesterol and sphingolipids, resulting in their thicker appearance.
Due to the change in membrane fluidity, within the membrane, this causes a flaw in the fluid mosaic model. As membrane rafts are more rigid, proteins do not diffuse in them as quickly as in the normal membrane structure. This is demonstrated by fluorescence recovery after photobleaching (Frap), as the bleached section of membrane does not completely recover after allowing time for diffusion to equilibrate the structure.