Israel was one of the first place to boost (the first?) and I think the rationale there was not just about antibody levels. There were multiple factors. The emergence of Delta which to some extent bypassed the previous two vaccine doses. The decline in protection against severe disease over time (and specifically against delta). And yes, even temporary control of the community spread (which is maybe the more questionable aspect) to allow the country to remain more or less open while keeping hospital load and deaths under control.

In Israel I also believe there is different policy for people that have been infected. This varies from place to place. Determining whether someone was or wasn't previous infected isn't that simple. You can do it if you have good contract tracing, testing, and more centralized health services like Israel does. Also many places didn't have a large enough portion of the population get infected in the first place so maybe not worth worrying too much about.