Stable coins increase convenience when working with cryptocurrency. It is convenient to withdraw funds from a coin or token, which began to fall in price. It is always a kind of buffer between cryptocurrency and fiat, or between separate types of cryptocurrency, if there are no direct trading pairs or funds, for various reasons, you need to hold it for a while until a certain date or event.
However, stable coins can not affect the stability of the price of cryptocurrency. They have nothing to do with it.