Neub question.
 

What do I build a market for? Only to employ people or are there other benefits?

Simply: you don't. You won't need it at all. :)

It does have benefits: employment. When starting a country, it can help you get better moral and more money. Later on, you will start making money some other ways (trade on market, loot opponents etc.), then this building will be less important.

Does market help generating money? How?

Quote from the help:

"Market Plays a main role in the employment, because they employ 50 people in contrast to other buildings and mines, which only employ 15. Only employed people pay taxes, so it is useful to keep the employment rate high. Important! If the unemployment rate is high in your country, it has a bad influence on the public mood."