Well keep in mind I actually have no reason to disable it, I was just testing the new UI feature which wasn't there before to see how it worked... (And I found an issue lol!)
But to answer your question; two things come to mind, if you keep WiFi 1 (802.11b) enabled you could be slowing down your whole 2.4GHz frequency if an older device supporting only the old standard jumps on the network and your router has to accommodate for it.
You enable B/G protection (which it is by default as you can see in the screenshot) but that involves more CPU overhead to support the older standards on the router than flat out disabling based on my understanding.
So what if I have no WiFi 1 devices at all, and don't want them on my network, but want to keep WiFi 3 (802.11g) enabled with B/G protection on?
In that use case it would work.
It could also be used to validate no presence of WiFi 1 devices on the network, you could use it for troubleshooting and identifying older gen WiFi devices on the network that get booted off, etc.
WiFi 6 or 802.11ax and its features absolutely supports the 2.4GHz frequency if the client does as well.
Same as the answer above, you could want to disable all 802.11b devices off the network; but want to keep WiFi 3 (G) enabled devices enabled.
But again I was just testing the feature, I don't actually have any 802.11b exclusive devices and I wouldn't care if I did on my 2.4GHz frequency, since in my setup it is mostly used for older devices and IoT, etc.
What he said!