Minecraft's world is divded into chunks. One chunk is 16x16x384 blocks: 16 wide, 16 deep & 384 tall.
Each chunk has a tag called
The Wiki says the following:
The cumulative number of ticks players have been in this chunk. Note that this value increases faster when more players are in the chunk. Used for regional difficulty
So why it is used is clear: it's used as a basis for the "local difficulty". Chunks with a higher local difficulty will have a higher chance to spawn more powerful monsters (by spawning them with better armor/weapons).
This tag is also very useful for an archival reason: with certain tools (like MCASelector) you can actually filter & delete chunks based on this tag.
So it is possible to delete all chunks that have an inhabitedTime lower than, for example, 10 minutes.
This is very useful when you want to regenerate unused chunks (because of a new Minecraft update) or if you just want to provide a world download.
What seems to be wrong, though, is what this tag is actually tracking.