Two reasons that matter.
SPEED
Newer versions execute substantially faster.
The difference on a busy site can be considerable, and it costs nothing.
If your site is slow and you are on an old version, that is one of the easiest gains available.
SECURITY
Versions stop receiving security updates after a period.
Running an unsupported version means known vulnerabilities are never patched.
That is the more important reason.
WHAT ELSE IMPROVES
Compatibility with current applications Access to features newer plugins require Better error reporting
WHY PEOPLE STAY ON OLD VERSIONS
Fear of breaking something An old plugin or custom code that requires it Nobody ever looked
THAT SECOND ONE
If something genuinely requires an old version, that component is itself unmaintained.
That is a separate problem worth addressing.
THE PRACTICAL POSITION
Move to a current supported version, after backing up and testing.
