Is it possible to play it off-line though? I remember that was an issue when I got The Orange Box with Half-Life 2.
There are often other enhancements, too, including native support for Mac, Linux, etc.
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.
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?