If a fighter has a high thrust to weight ratio, it can perform evasive maneuvers better, if a bvr missile is heading towards it. Evasive maneuvers like tight turns and rapid altitude changes requires a good Thrust to weight ratio.
For example, those huge long range missiles like the phoenix missile was only effective against not-very-maneuverable bombers. Against agile fighters, it would mostly miss its target. If a performance of a jet was not a consideration, then phoenix would have been effective against fighters as well.
So, for example if a bvr missile is heading towards it, a fighter performs 1.jamming along with 2.evasive maneuvers & 3.countermeasures, and ditches the missile. But now after ditching the first missile, the fighter which doesn't have a good thrust to weight ratio, has depleted its energy and cannot perform its evasive maneuvers. If a second missile comes in now after a few seconds, the fighter now only has 2 of the 3 defensive mechanisms available now, so the chances of the fighter getting killed is higher in the second missile. I don't know about other airforces, but the Russian and Indian Airforces launch 2 missiles within a few seconds of each other as a standard practice, against a single target.