Imagine you want to compile the javascript code of CkEditor into a binary application. Just assume this is possible. Then you would not ship the source code of the library any longer.
If you now even make modifications to the source code to distribute within the binary, you do not satisfy the LGPL as it requires that you provide the source for the library along with your binary. Same for the MPL.
If you don't want to distribute (your changes in) the source-code form of the library, you then can get a commercial license by the project (you buyout the license).
Next to such specific cases I can imagine that some folks just would like to have a commercial license as form of a backup if their legal department is too much puzzled. If the license is relatively cheap, they can opt for it and continue with their own work w/o being further interrupted. Business, you need to keep it running.
But regarding the LGPL, as long as you offer source (incl. the changes you probably make) of the library, you're fine to use it with non-free programs. So this does not mean that your website must be LGPL it's just using a LGPL'ed library.
