We need altcoin because it is a good alternative investment rather than bitcoin because there is a lot of good altcoins in the market that can give a lot of profit or even a higher return compare to the potential return of bitcoin so it is a good step to just spend some of our bitcoins or money to altcoins.
Actually, more profits mean more risk, this is the rule. You can't deny this truth. You earn a lot of profits from an altcoin, and at the same time you make that profit, someone also loses the same amount of money you received in that coin. For me, Altcoin has the role like its name, "Alternate Cryptocurrencies" for Bitcoin. 