250 watts is an output a fairly average amateur racer will produce at threshold (where threshold is their one-hour max sustained effort). The pros are significantly higher than that (estimated 350W or more for the latest batch of 1-hour records).

But, an extra 100 watts for even a few seconds can make a significant difference. In cycling, staying in the draft is a major factor. If that short burst of assistance helps a racer keep the wheel of the guy in front, it conserves a lot of energy for later in the race.

The one hour record is actually about 470W from Wiggins.

(Though not a record of power for a single hour, just distance ridden on a track in one hour. But it ends up measuring more or less the same.)

Damn, the 350w was Jensie's estimated output (or maybe it was 375?). I knew Wiggins went substantially faster, but didn't realize it translated to that much more power. It's really nuts what these guys can do.

Watts per kilo is what they all optimise for in pro cycling (expect for the pure sprinters where it's no as important).