On earlier, low-powered B737 such as the -200, flap 2 provides almost optimum takeoff performance on longer runways. Flap 1 is also available for takeoff, but requires a very long runway and high speed tires.
On the later models, it was not favored to use either of these settings due to the risk of a tail-scrape (due to the longer body length) so generally flap 5 is the minimum used by most operators, though I think the -500 was OK at flap 1 (short body).
So why is the flap 2 gate still there on later models? Probably because Boeing have a philosophy never to change a part number unless there is a clear cost saving in doing so.
And in the event of a flap malfunction on landing, I would rather have flap 2 than flap 1 if that was available.