It's supposed to reduce field 'flicker'. The field of a 625 line TV runs at 50Hz, giving 25 pictures a second (interlaced).
In a 100Hz TV it digitises the picture, and stores in in memory. It then plays it back at twice the speed (showing each field twice), so any potential 'flicker' is eliminated.
My personal view is that it's a waste of time, I can't see any flicker on a 50Hz set, and the picture quality is usually less on a 100Hz set due to all the extra processing.
As a TV service engineer, we get a great deal more complaints about 100Hz sets than 50Hz ones. They are particularly poor if the signal is a bit weak.