Why did the plantation owners think they needed slaves.

Since they were farmers, they need the slaves to collect the crops and to plant more. Most of the time, they lived on large acres of land so it was more than a one man job.

They are plantation owners which means they're gonna need people to work in their land and most plantation owners owned acres and acres of land which was clearly a lot of work for one man and so they needed more men to work on it and so they got slaves. (it also somewhat showed that they had a lot of money since they can buy slaves.)