A market maker is primarily there to provide liquidity for an exchange. Market makers solve the issue of insufficient liquidity by placing orders in the market on both the buy and sell side that can be taken by traders on the exchange.
Market makers make sure that the order book is full of suitable and appealing orders at the current market price. Without market makers it would be very difficult to maintain a healthy order book.
It is also worth pointing out that at GO.Exchange, while we do have market makers, we make sure that the market makers don't influence the volume on our exchange as we believe transparency and real, verifiable volume is of the utmost importance.