nothing can work alone in this world, so does bitcoin, even though bitcoin is the number one cryptocurrency for now (in 2017), bitcoin still needs other coins to do daily work, for example XVG and XRP can send quickly without having to wait long, while bitcoin has to wait hours for one transaction, what if there are hundreds of transactions at a time?
it can be said that altcoin and bitcoin are brothers who cannot be separated, because both need each other.