It's pasteurisation, your 50C-only yoghurt would potentially not last as long; possibly even succumb to unwanted bacteria before (or the wanted be outcompeted by the unwanted) turning to yoghurt.

I don't measure it to 80C, just heat milk until it breaks then switch off and let it cool. I didn't have a thermometer when I first made it, but now I might measure that it had cooled enough. I wouldn't say it has to be as hot as 50C, you just probably don't want it hotter, so if you're making it without just let it cool to tepid, feeling warm to the touch, then add whatever you're using. Cheese similar, just more specific cultures.

(And I suppose if you really get into it, the specific temps and holding them thing is a lot more true for cheeses - or rather for different types - than it is for yoghurt.)