System 6.0.5 was released to support the Macintosh IIfx. If someone wanted to run the original release software from the IIfx, that would be a good reason why.
Additionally, there were bugs and glitches introduced in System 6.0.7 (which quickly replaced 6.0.6 for a show-stopping bug), that weren’t present in prior System software versions. Additionally, System 6.0.8 introduced a few more problems that weren’t in System 6.0.7.
I ran System 6 at home and used it at school between 1990 and summer of 1993, and even though I had System 6.0.7 and 6.0.8, I found (on my Mac Plus at least) that the 6.0.4 it came with was more stable / reliable. I never had 6.0.5 in those days so I can’t vouch for that version specifically.
These “point” releases contained slight tweaks / fixes to system software and also brought support for new machines. That was the point of 6.0.5, to support the IIfx. System 6.0.6 (replaced with 6.0.7) was made to support the Classic, LC, and IIsi. System 6.0.8 was made to integrate some of the networking / AppleTalk / print functions in System 7 back into System 6, as there were a lot of enterprise and educational customers still using System 6 after the release of System 7.
System 6.0.8L was released as a modified System 6.0.8 for support for some machines that were released AFTER system 7, but where Apple either didn’t have support baked into System 7 yet or there were specific customers who needed a System 6 version for the new systems.
If you thoroughly use each point release of System 6.0.0 through 6.0.8 you will find differences and some compatibility issues with software running on one and not running on another. Some of the specific changes to the OS broke software in strange ways. So it wasn’t always best to have just the latest version only. I myself had multiple boot floppies with different system software versions just for that reason.