The benefits to fast lenses aren't limited to massive telephotos. Last year I purchased a slightly used Nikkor 20mm f1.8 specficially for a trip to Iceland and a chance to photograph the northern lights--even though I have slower lenses that would cover that focal length. I was rewarded with some spectacular shots, virtually all at f2, eight second exposure, ISO 1600. Equivalent exposures for a slower lens would have required a substantially higher ISO, and a lot more noise in the black expanses of the night sky and the subtle colors of the aurora borealis. Longer exposure times were not possible, as that would have turned the stars into streaks. In a situation like this, a generous f-stop was my only viable option.