L5 used to be the first terminal level, meaning that you could stay at L5 for as long as you liked as long as your performance was good. (Nowadays that so-called "growth expectation" has gotten a lot fuzzier, with some orgs letting people stay at L4 or even L3 for many years.)

So L6 was the first elective level. Anyone who reached it did so by opting in, rather than just going with the flow. That is why it carries a bit of weight.

All this was more true back in the 2000s. (Source: me, ex-Googler, 2003-22.)