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?
There are often other enhancements, too, including native support for Mac, Linux, etc.