Well so first of all ETLegacy is a game and a mod, you don't need ETL to play on servers running Legacy mod. The benefits of ETL, on the top of my head, is that it works on much more devices than 2.60b, for example Mac OS X Catalina, Linux 64 bits, Raspberry Pi, Android and even PS Vita.
It also fixes the pain points of ET with built-in minimizer, auto desktop resolution, etkey generator.
Now, if you are using Windows 10, ETL will not give you any substantial benefits over 260b if you are using Nexus. I have benchmarked ETL 277 and ET 260b, and 260b gives me higher framerate, which matters if you are using Nexus. You can use ETWC to mininize ET, I added an etkey generator in Nexus, and it is very easy to use a custom resolution with cvars on 260b. Also ETL does not support ETPro / Punkbuster.
Nexus does not support ETL, but Legacy mod via 260b.