The Isle of Man TT is a time trial event, meaning that riders are ultimately competing against the clock, not necessarily against the other riders on the course.
Usually when a rider has caught another rider on the course, it means one of them overcame an initial 10 second separation, and thus is considerably faster than the other.
The general protocol then is to let the faster rider through. This benefits the faster rider, who clearly has a superior pace to the slower rider. But, it also benefits the slower rider, who can get a tow from the faster rider, and benefit from their added confidence on the course.
As such, it’s rare to see riders get too close to each other on the Snaefell Mountain Course. Then again…this is motorcycle racing, and these are motorcycle racers…