Most of such check are performed because a software author put some macro into autoconf.ac, sometimes without good reason or without any thought at all. I see

Most of such check are performed because a software author put some macro into autoconf.ac, sometimes without good reason or without any thought at all. I see this such attitude in many different areas e. g. people copy-paste some config options into software configs from some outdated how-to or StackOverflow and OK with now knowing what a given option does and if it is relevant in given case. May be with autotools it happens more often than with cmake (though I've seen enough bad CMakeLists.txt too) because there is a myth that autotools is very hard to learn so developer don't even try to read documentation and just put some random stuff into their .ac/.am files until it works for them albeit slow.