Altcoins prices push the price of bitcoin up and down !!
if Altcoin is a simulation of bitcoin's path, so why not just bitcoin enough ??
Any altcoin owns its name, project and ideas, so why that altcoins are related in the pricing with bitcoin ??
if you are saying why altcoin exist, it means you are automaticallysaying why Ethereum, why Bitcoin Cash etc etc. bitcoin has it is purpose, that is why it came into exist, while it purpose is been neglected, Bitcoin Cash came into exist to correct some of these error.
every altcoin has it unique purpose of been into existence just like bitcoin. and the existence of altcoins has nothing to do with the bitcoin market fluctuation. this is one thing every decentralize currency will always experience "Market Fluctuation"