I think the main purpose of all those currencies is to create speculative instrument, which, if traded on 2 or more exchanges, may have arbitrage potential also.
And also to serve as a testbed for new features and ideas. Although sadly that part tends to be overlooked in favour of the speculation side. It will be a shame when some great features get implemented in an alt, but everyone ignores it because someone dismissed it as another "crapcoin pump and dump". Everyone seems to forget that not every opportunity has to be a financial one.