Freeze button can be used for many many functions.
For example, just yesterday, I had a singer request a piece in a lower key than I play it. Set the transposition, then freeze it. All the registrations for the song work, and but it stays transposed.
Or, when recording a registration, holding some parts the same to 'copy' them from one registration to another.
Or, when you want to audition styles, but want to hold the tempo the same as you check them out.
And other's I can't think of, at the moment.
Adam