it's a just-in-time cost saving measure. but you have to plan for it from the very start of a particular service cause it's basically impossible to retrofit. but yeah, my read is that it was the hot fad for a couple of years and faded away - AWS Lambda and the like still see use but it's on fewer buzzword checklists.

the other advantage (the valuable one) is that it forces you to treat running processes as disposable. so rather than having a couple of bespoke applications that are very hard to upgrade, you have short-lived applications that get turned off all the time. you'd be surprised how often this is a problem in the large companies. makes actually releasing code for use by end-users unnecessarily hard. this approach is one way to solve that problem but imo there's better ways to achieve the same thing, without so much of a cost in terms of ux.