Requesting additional track miles could be because the aircraft wasn't yet configured for landing, perhaps because ATC held the aircraft high due to traffic, or because the pilots didn't manage the decent well.

There are also maximum landing weights (which are different from max takeoff weights) which could potentially be a factor.

That doesn't seem to be related to padding. Airlines still like to arrive early. If there's no gate available, it's cheaper for the airline to wait on a taxiway than to circle in the air. Fuel costs are a major driver of airline profitability, hence airlines have no incentive to spend time in the air if it's not absolutely necessary.