Yes they are needed because each Altcoin has it's own technology. So we don't need to rely on just one. From an investment point of view, Altcoins give out larger bursts of profit in a shorter time, whereas Bitcoin gives out profit in a longer period.
Some actually risk on altcoins for years and roi is not as much as Bitcoin... Right now, I think Bitcoin has already grown in price so much that it will not be able to replicate a similar one from last year though.