Most altcoins use the same or similar technology as BTC, for the market to develop and grow we need diversity, it can finally lead to better solutions and to a fster way of solving the existing crypto problems. Alts bring their own value to the market and add to a wider adoption of crypto by general public. I might sound very naive, but we won't move far if we all hold only BTc and consider other coins only from the "sell as the price goes up" perspective. What's more, I do think that with time some of altcoins can overtake BTc as the market leader, I'm not saying it's going to happen in 2019 or even in 5 years time, but this opportunity can not be dismissed. So I do keep some of the coins I consider promising in the long run and I hope for more good alts to appear