Price of bitcoin is a big factor in traders since they buy in a lower price and then sell it for a higher price.
of course it matters. Alt coins and exchanges sites base their prices on BTC. so to determine the price of alts you will have to take a look at the bitcoin price. Lets just consider BTC price if somewhat the USD of fiat currencies.
If BTC price shoots either up or down, the alt community will either dump their coins or hold on to their coins. BTC is the basis of all alt coins price.
Let's just assume that BTC's price goes down to 1$ overnight, alt coins will have very low value as well. Making the bitcoin community in panic state.