In my experience, there are two reasons why a prospective employer may wish to have a sample of your handwriting, one legitimate and the other questionable.
a) They wish to see whether you can write legibly for company purposes and also satisfy themselves that you genuinely completed your own application form yourself. This is entirely legitimate. (I was always taught to include a covering letter written in my own handwriting with any application).
b) Some companies like to submit your handwriting to a graphologist.
(i)In my view, this should never be donewithout telling you.
(ii) Graphology is a load of cobblers anyway. So far as I know,its alleged validity is unsupported by any hard scientific evidence whatever. I suggest you should avoid tying your future prospects to these companies as they have probably used a crystal ball and Tarot cards to produce their strategic business plan.