Moreover, going to the Settings app is laborious. There's a lot of stuff there. When you open it, you don't always "start from the start", so that pulling it up requires working out where it is in the app you are in order to navigate appropriately. Instead of focusing on the task of disabling wifi and going somewhere, you have to focus on the task of navigating an app. And also if you have a phone that wasn't bought yesterday, it will probably close some other app you were running because it's run out of RAM.
All so that some weird usecase I can't understand is available. This fellow is the only person who's described a usecase for disconnecting from wifi networks while leaving wifi on, insofar as leaving the radio on won't have upsetting results for him. But the feature doesn't work for him.
For me, sometimes I need my battery to last all day not just till dinner. I want the radio off.
I can't think of a non-technical/non-exceptional use that is benefited by this feature.
(NB. I quite like the idea that my wifi will spontaneously turn back on again when the battery is charged adequately, or at 5am or some such. It would've saved my data on numerous occasions. But that's a different feature that hasn't been implemented.)