Why does displays have separate spaces prevent the spanning of a window across more than one monitor?

When this feature first came out I immediately disabled it because I noticed you couldn't stretch windows across two screens. I thought I might try putting up with it, but at the time I found it made it damn near impossible to use Avid Media Composer effectively.

It's been disabled in all the years since and I've never thought about it since until just recently where now twice in one month I've come across situations where something only works if it's enabled. One is for disabling the behaviour where fullscreening a video makes a second monitor turn black and now with Sequoia, the ability to use the native window tiling they just introduced.

I've always used magnet and then later rectangle to achieve the same and will probably just revert but I thought I'd try and give the native option a shot. I checked to see if that stupid shit with not being able to span windows has been fixed in the intervening period but it hasn't. Does anyone know if you can make it work with separate spaces enabled or at least why it works this way?