Turn off Ambient Occlusion in pretty much any game, and youll understand why shadows are important.
Good shadow maps are also a god send, I dont think you "need" RT shadows if youve got really good shadow maps, it just gets more expensive than RT when you are doing things like foliage.
Reflections are hit and miss you can get great reflections without RT but well implemented RT shadows a re something else.
Global Illumination is something else, I think Enlighten was a good implementation (look it up if you dont know about it) but it had its limitations.
I get that people say RTGI isnt that much better than Enlighten, but as a developer there now way devs are gonna sit there saying well its good enough no one will even notice if we go higher.....they will keep pushing forward and having a solution like Lumen or other Real Time GI also makes life easier for the dev.
^Yes you could manually light the scene so the need for GI doesnt exist.......or you could use GI and not spend days trying to correctly light the scene.
If you have the money/time you could do all this manually.....see Naughty Dog with how they lit the real time cutscenes, their lighting artists would manually place lights as the cutscene is going, not every studio can afford that kind of investment.
And even with Enlighten Dynamic GI, you get errors in lighting, maybe not perceptible to the average user, but when you are pushing the boundaries of technology there are people who notice the inaccuracy, I pointed this out in another thread about Horizon, when you enter a dense foliage area, the game looks beautiful no doubt, but once you spot the lighting is inaccurate, you can never unsee it.......sometimes I do regret having gone into offline rendering because now in alot of games i spot those errors/mistakes and cant unsee them, my eyes get used to it quickly enough, but still i always imagine what if they had an option for more accurate lighting/shadowing.......
Alas diminishing returns are a thing, we are reaching a point where the advances from a technological perspective are monumental, but the visual returns arent as great for many people.
See Glass in Unreal Engine.....or any Engine for that matter, I dont think theres any game that has shown me impressive glass.