Why would a commercal flight circle while enroute?
As others have noted, this is called a holding pattern, which is used to delay an aircraft while in the air. Of course, this takes time and fuel, so the system works to prevent this from being necessary.

The primary use of holding patterns is by ATC to delay arrivals. This could for traffic reasons, to delay the aircraft until it can join the line of arriving aircraft. It could also be for facility reasons. Bad weather can reduce the rate at which an airport can accept aircraft, meaning some will have to enter a hold. An airport can also be closed for weather, or for another reason, in which case all aircraft must enter a hold, or land at an alternate airport.

Pilots sometimes need to use holding patterns as well. If they have an issue with the plane, they may need time to follow their checklists and diagnose the problem. ATC will give them a place to hold for this time.

Standard arrival procedures generally have holding points published for these cases.  When a holding pattern isn't defined, one can be created. The aircraft will fly for a standard time before turning around, and repeating as necessary.

A better FlightRadar example is perhaps the following image, showing three aircraft in the stack and one leaving it



Source