Diversification is KEY when investing. Lets say you put all your money in bitcoin which is doing really well at the moment. Some crazy FUD could wreck that market for a while and you are stuck holding this coin for a long time waiting for it to appreciate again. But if you diversify, only some of you money will be temporarily lost (Depending on if you HODL or sell) in bitcoin and your holdings in Ethereum could easily go up. Never put all you eggs in one basket.