For cosmetic reasons only.
You see a great glamour somewhere, and want to copy it, but it requires a different job AND race. You don't want to fantasia your current character, or create an level a new one, with all it's disadvantages.

So, how about letting us change to a different race via fantasia in another job, and have it be effective only in that job (and any others you select). That way, you can have even more glamours unique to new race/job combos, and SE gets to sell lots more fantasia potions...

However, with all the options open to one character, you'll probably run out glamour storage space.