Having another cassette makes wheel swaps faster and cleaner because you don't have to touch the cassette.

The cassette you use depends on the terrain you ride, your fitness and your pedaling style (spin vs mash). Only you know all those factors. If you feel that your current cassette does not have low enough gears, get a new one with lower gears. If you never use the 28t cog then get a 12-25 or similar. If you find yourself often wanting a higher gear than you have, consider an 11-xx cassette.