In my experience it’s because they’re drinking 18 of them and don’t want to be sloppy drunk after 10-12 hours. I prefer the more direct method of hard liquor to be more efficient but I get the thought process.

It's to beer what LaCroix is to fruit juice.

Maybe they're keeping a better balance of hydration to alcohol for their old brain, and not having a fruity or sweet taste but not boring plain water as well. Lite beer can occasionally be refreshing. Although personally I'd rather pour half a beer into a cup of ice than drink most light beers.