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.
As one of authors of Heroes III one I could say Xash3D is amazing project worth to check for Source 1 game fans.
Valve support for their old games is outstanding and they all have native support, great performance, constant updates. It's like extremely rare for old games to be supported that well.
HL1 is already quite performant though and does have native support for Linux and Mac on Steam without needing third-party engines.
Not disputing that engine replacements / source ports can be great (I work on one myself) but it's weird that there are three replies that sidestep the actual question: What does this particular engine replacement add over vanilla GoldSrc?
