Yes, their main purpose is to have different colors on different lights. However in the vast majority of cases (if not always) you simply cannot reproduce this setup in post.
The human eye is quite good at detecting natural light falloff and it will detect the things which are Photoshopped, especially if we talk about a setup with multiple lights (we include here the ambient light as a separate light source) and uneven surfaces.
Also, having the gels in-place you can tune up your scene with great accuracy in order to obtain the mood/message you want, including adjusting your subject(s).
Again, having real gels will give you the possibility to re-shoot the scene in different ways (different angles, subject positions, etc.) in order to have options from which to choose.
Generally speaking, the old rule still stands: Get as much as you can straight off camera and use post-processing for minor adjustments.