Why was there need for slaves?

There wasn't.

My versionSince the first city state the prosperity of the rich has depended on cheap and dependable labour.

In the early days of the USA for example, the use of slaves lowered the cost of farming to the point that the landholders could make a profit.

People who did not own slaves could not financially compete and thus could not become more wealthy.

So the need was driven by the financial need to make a profit.

UNTIL the whole economy had to stop the use of slaves at once, the fact that it was legal and your business would not be viable made the "NEED". Once everyone can not own slaves then the NEED goes away.