The values written in JPEG files are arbitrary and essentially meaningless. They don't relate to anything about the camera, its sensor, or the resulting images. They certainly don't relate to image quality or acceptable resolution for printing. Really, they mostly serve to confuse people.

The EXIF standard seems to imply that if the tag is missing, 72 is the (still-meaningless) default. However, it is apparently mandatory for the TIFF standard, from which the JPEG/EXIF format basically inherits everything. So maybe it has to have some value to properly comply with the standard.

Others have noted that some desktop publishing or word processing software reads this value and will use it for default scaling on the page. So, I guess in that sense, there is "meaning", but I'd argue that this is really misapplication, because the original value doesn't have meaning. Garbage in, garbage out, as the saying goes.

Now, maybe in an alternate universe the standard could relate to a standard print size. Or, the camera could estimate real detail in the image and give a recommended maximum print size. But none of that is the case. In practice, these values are meaningless and you should ignore them, even if some software makes assumptions based on them when opening files.