why kill the tabs to have a browser without content running in the background?

It doesn't kill the tabs, just removes their content from active memory.

If you have private tabs open and the fucking OS kills the browser, then those tabs are gone and you need to reopen them from scratch, log in again etc. With this option enabled, the browser can free up more memory so the OS is less likely to kill the browser.

Now, with some phone brands it might not matter as their background process culling is more aggressive, but with default Android there's a difference.

But it's just an option, idk if it's disabled by default. Maybe there's an extra config option for it somewhere.

Yea FF on Android does take up more memory than Chromium, maybe because Chromium can share more Android core things, idk. Either way, if you run out of memory, the browser will start flushing things regardless, so it doesn't crash.

Chromium also does a different thing where it still shows the unloaded website but in "preview" mode and you need to refresh it anyway, which I find even more annoying.