Essentially ULWGL will allow you to run your non-steam games using Proton, Proton-GE, or other Proton forks using the same pressure vessel containerization and runtime that Valve use to run games with Proton
This is the crucial piece of information. In less technical terms: Proton is designed to run in a very specific environment and it might be incompatible with your system. Steam runs Proton inside a bubble so that it interacts less with your system and so the incompatibilities don’t become a problem. ULWGL aims to create the same bubble so it’s the correct way to run proton.
Imagine you were asked to start speaking a new language, eg Chinese. Your brain happens to work quite differently to the rest of us. You have immense capabilities for memorization and computation but not much else. You can’t really learn Chinese with this kind of mind, but you have an idea that plays right into your strengths. You will listen to millions of conversations by real Chinese speakers and mimic their patterns. You make notes like “when one person says A, the most common response by the other person is B”, or “most often after someone says X, they follow it up with Y”. So you go into conversations with Chinese speakers and just perform these patterns. It’s all just sounds to you. You don’t recognize words and you can’t even tell from context what’s happening. If you do that well enough you are technically speaking Chinese but you will never have any intent or understanding behind what you say. That’s basically LLMs.