People become bored and [hence] less productive doing the exact same work -- moving them to other areas of the line improves productivity.

There's an ancillary benefit in reducing friendships which can be a productivity sink, profit-focused businesses want people to be friendly - committed to their colleagues - but not spending all day chatting.

Also moving people improves "bus factors".

Having people better trained to do more things is better for everyone- employees and company. People are going to form friendships regardless of whether you rotate or not, and you're going to be bored doing factory work, hands down, no doubt about it. Different people have different methods for getting through it, but you can't not be bored even with a different position every day.