Mammoth Oak is a small brewery that opened during the pandemic. Guest taps were 50% of the tap array, although on a previous trip were maybe 20%. I figure a start up operation needs a few backup kegs in case something isn’t ready on schedule. And as you say, at most places it’s a way of introducing customers to something they haven’t encountered before and might want to explore. I saw something similar at Salty Oak ( no relation) in Panama City when they were just getting started. Gradually I expect fewer guest taps and more home grown product. But all but the largest seem to have guest taps. I saw none at Sierra Nevada or Abita breweries for example.