As with pretty much any instrument, it's all about getting rid of meaningless noise
. Especially for a handheld, instantaneous measurements can be full of inaccuracies. Is your hand moving? How fast? What direction? How about your platform (boat). Going up a wave face, down one. The selection of which time period should be based on what you observe about the measurements you are taking and what cycles you can discern.
Curious about the actual periods they provide though - 5,10,13. Those are not any standards that I know of in the meteorological world. The "3-second gust" is used very widely in engineering design and met. record
keeping. From there the next standards are almost always 1 minute and 10 minute. So, the 5,10,13 seem like they were selected somewhat at random to allow you to remove signal noise
. I'd love to hear if it was other than random and what the reasoning was.