Alt-coins provide an easy store of value when speculating for example. Many exchanges allow BTC/LTC currency pairs and while BTC is volatile LTC and FTC seem to hold steady around 2.1 $ and 0.0004BTC respectively. So when BTC is low you buy BTCs in cash and then when it spikes you sell it for LTC. Then you remove the necessity of cash from the trade, which helps given the difficulty of getting fiat currency into the exchange quickly. So it's not BTC that needs alt-coins, it's just that alt-coins make it easier for BTC trading and eventually supply.