Bitcoin has already a large amount so the percentage of profit you can get from it would be lower compared to having altcoins which has a chance of growing in terms of percentage but still depending on the market capitalization. Some would just buy more altcoins and then later on exchange it to bitcoin. The disadvantage you have with altcoins is that when it dumps, the percentage drop is bigger as well compared to bitcoin that's why it is a high risk, high reward trading when you decide to trade or invest in altcoins.