It's a whole different engine that may have some advantages that make it worth checking out over or in addition to the updates Valve has made for this special r

It's a whole different engine that may have some advantages that make it worth checking out over or in addition to the updates Valve has made for this special release: improved performance (through a more efficient culling system, etc), support for modern features like dynamic lighting, use of HL mods on newer systems by providing compatibility fixes. The engine can also run other games and works on mobile devices with less common screen resolutions, but YMMV. For a while, it was a good fallback to the official game if you were having a problem with it on new or less-supported hardware, but with this anniversary release, maybe it is less "necessary" and more "cool" to use an OSS alternative engine.