A similar older question already exists, but even that has no definitive answer. I hope somebody can give a clear explanation and answer with regards to macOS Mojave and above, and my doubts below.
I use Spotlight only to open applications quickly and not for searching.
For sometime now, there have have been some issue with spotlight indexing not hapening on some drives properly, slowing down or hanging up both Finder or spotlight or the whole system because of it. So I have finally disabled it on some volumes out of frustration (I tried everything - deleting the index, reindexing it etc.):
$ sudo mdutil -i off /Volumes/Backup/ /Volumes/Backup: 2020-12-01 05:02:08.758 mdutil[848:47342] mdutil disabling Spotlight: /Volumes/Backup -> kMDConfigSearchLevelFSSearchOnly Indexing disabled.
Disabling indexing has made my system, and especially Finder, more responsive. I can still do all file search using Finder. So what exactly does spotlight indexing do? Is it only to enable searching for something through Spotlight? What disadvantages are there (if any) in turning it off)?