The two main advantages of the OLED deck are a drastically better screen, and much better battery life in demanding titles.

Games that max out the Deck's power draw (Cyberpunk, Bladur's Gate, etc) will typically get about 1:30 of battery life on the LCD deck, and 2:30 on the OLED deck, which is a pretty huge increase. It's less of an issue for less demanding games, and you can always get a high output battery pack instead.