Altcoins provide more successful projects than the pioneer and hence have features which Bitcoin lacks.
Monero provides complete anonymity in its transactions making them more secure and keeping the identity of the sender and receiver private.
Liteccoin has a blockchain of 2.5 minutes which is four times faster than Bitcoin and hence makes transactions faster.
Ethereum is not just an investment asset but can also be used as a token for smart applications.
Technically altcoins have their own platforms and their own goals which is not present in bitcoin. In financial reasons, it has the advantage of giving people and traders options to earn and an alternate way of income, although lots of reports regarding about some failure and losses, but it is a total package because you've chosen to do that.
To supplement your answer altoins give the opportunity to join community around this coin like joining Bitcoin community few years ago. Ok, they don't have such great potential but I mean the early stage.