Take your brand new, never run, chain and lay it out on a flat surface and measure it, end to end. Then stretch it as much as you can and measure it again. Record both values in your log book, just so you don't lose track of it. Run the chain a few races and repeat - you will find that the chain stretches more than before, and, over time, it will keep growing like that, and the wear will start eating sprockets after it grows a bit.
No, I did not do that test all the time, but after doing it with one chain several times and watching the growth, I started treating chain like the expendable item it is, using one quality chain for maybe half a season, no more, then trashing it, and sprocket life improved a bunch.