“You might as well play the game” assumes you own the game (and have people to play it with consistently) or have access to it in some context where you could try it out. A lot of what they (and other playthrough videos) cover is either before release or shortly thereafter which means most people won’t have the game. And while you are free to differ, I find seeing the game in play much more informative than someone passing brief impressions through a filter of their wants and needs which frequently have little to do with mine.
But even beyond that utility, I enjoy watching people play games I like and it can feed some of that craving if I’ve been having trouble getting them to table, or perhaps have been having a boardgaming dry spell (I’ve only managed to get in two games in the last five weeks, and they were Wednesday and Thursday of the same week). Or for example, I’ll sometimes watch campaigns of games I’m moderately interested in but aren’t something I personally need in my collection (e.g. Asassin’s Creed, too many minis and too high a price tag, plus I missed the KS, but it’s been fun seeing other people tackle it. Though now they’re talking about a retail version that will have no minis (or maybe just player minis) and be under $100…so who knows.)
Just the perspective of someone who almost exclusively watches playthroughs, when it comes to boardgaming videos. (I do watch SU&SD reviews but mostly because they do jokes, not because I care about half the games they’re reviewing or agree with their feelings on half the stuff I am interested in, c.f. their burncycle, Etherfields and Sentinels of the Multiverse reviews.)