Why are metal chains sometimes paired with a smaller metal chain?

I’m not at all certain what else you’re referring to here, but you may be talking about something like a chain fall. That has a heavy-duty “load chain” which is rated to handle the full load that the hoist can safely manage, and it also has an endlessly looped “operating chain” which the operator continues to pull in one direction or the other to gradually raise or lower the load.

The operating chain frequently loops around the load chain from time to time as it is spun by the operator (or twists in repose), but it is in no way holding any part of the load.

That must be it, thanks :)