The main purpose of money is to have some kind of measurable unit that a society will accept as having value, and which will be used to conduct transactions and exchanges (e.g. paying for goods and services).  Today, most societies use physical and electronic fiat cash and currency as money, meaning that the money itself in a vacuum has no practical use or utility for humans, but it is used because either a government has backed it, or people have agreed on the value of said money.  In the past, metals such as gold and silver were more commonly used as money.

The main reason money is needed is that it provides the convenience of not having to conduct all transactions or exchanges via barter, where different people will place different valuations on different items in accordance to how they perceive their utility of those items.  The larger the society, the more quickly barter can become a problem.  Barter is considered to be inefficient in an economy, and so money is the answer to help make that economy more efficient by providing a unit of value that everything else can be measured against.

What makes money a contentious issue though, is the fact that in many countries/societies there is a large concentration of money and wealth in the hands of a small proportion of the people.  This phenomenon takes on many names, including income inequality and the wealth gap.  In addition to the social unrest that can be caused when a massive wealth gap exists in a country (which makes the rounds on the news a lot in these days and times), such inequality is believed to also hinder the economic growth of said country.

So as you can see, money is essentially an "evil necessity"; it can be a contentious issue that causes a lot of debate and unrest, but at the same time, society needs money in order for the economy to run efficiently.