Why you need coins?

Answer 1

I assume you're asking the question "Why do we need coins as well as bills?"

Coins are less expensive to produce than bills for small denominations. Low denomination currency is used a lot in making change, and bills would wear out too quickly. That's why there's increasing pressure to stop making $1 bills and replace them with a coin - it would save $500 million to $1 billion in production costs each year.

Coins are easier to validate in vending machines. Bill readers cost about $400-$600, electromechanical coin acceptors are much cheaper.

Coins are easier to count. Bills have to be sorted manually (at least U.S. bills that are all the same size) but coins can be sorted mechanically.