To further explain:

1) If you use straps properly, by cinching and un-cinching at every stop light the old school way, it would be a pain in the butt if you accidentally opened the clasp too much and the strap slipped through. This would mean that you'd have to get off your bike and thread it back through again.

2) People's feet are different widths, so are pedals. So, a foot/pedal combination might result in lots of excess strap. Plus, there is only about 2" of the strap that is used for slack to cinch/un-cinch. The rest is extra, so many people cut it short...but this might contribute the the problem above.

3) Older, broken-in straps are sometimes slippery and may not give a good grip when cinching. The button provides a grip spot.

Buttons allow for short straps to be used without issue while being easily gripped.

But, it's wasted on 99% of people on the FG scene who don't cinch/un-cinch. Plus, you can't tuck the strap into the clip when there is a button on it. So, save your money.