I understand that bid/ask prices and bid/ask sizes are ultimately determined by the market maker. I am curious about some general principles that the market maker follows when determining the above parameters. If the market maker is just a middle man, why should he take pains to figure out these values so long as he is collecting money every time a trade happens?

I understand that bid/ask prices and bid/ask sizes are ultimately determined by the market maker. I am curious about some general principles that the market maker follows when determining the above parameters. 

If the market maker is just a middle man, why should he take pains to figure out these values, so long as he is collecting money every time a trade happens?

I understand that the bid price, ask price and also the bid size and ask size is ultimately determined by the market maker. I am curious to know what are some general principles the market maker follows while determining the above parameters. Not in depth algorithms, but general guidelines. The reason I ask the question is this: market maker is just a middle man and so long he is collecting money everytime a trade happens, why should he take such pains to figure out bid size, ask size, bid price, ask price?

I understand that bid/ask prices and bid/ask sizes are ultimately determined by the market maker. I am curious about some general principles that the market maker follows when determining the above parameters. If the market maker is just a middle man, why should he take pains to figure out these values so long as he is collecting money every time a trade happens?