Newer ones support newer hardware, or have been updated to better support. Older ones sometimes work better with older hardware,,, sometimes...
The incremental updates typically patches or fix something that may not work 100% right...
If 4.15 is working great, 5.1 could work same, better or not at all... Will all depend on hardware, or atleast alot of the time...
So CPU, chipsets, wifi, bluetooth, mouse etc...
You can always install and try it... But know how to get back..
Have a backup
If it doent work, you can revert to another using advanced on boot...
For 4.15.19 or 4.15.39 are incremental, the old ones are ok to remove. I like to keep a version or 2 around just in case...