To me it is a benchmark of your fitness/mental attitude, a bit like running a half/full marathon for runners.
100 miles is a long way to ride a bike – it is 6+ hours for people of average fitness, and quite a challenge.
I did RideLondon last year, and found it very tough (42, overweight, not trained enough) Garmin clocked me at 6.40 (moving time)
I’m doing RideLondon again this summer, i’ll be a couple of kilos lighter than last year, and I’ve certainly trained a bit more.. I’d love to get under 6 hours (Moving time) but in reality i’ll just be happy to beat last years stats.