IIRC you should be able to download it and then put Steam into offline mode and it won't call home until you flip that toggle again... unless they changed how that works since I last fiddled with it. Might need to play once before flipping toggle off.

Its worth it for the assets alone, you can play hl with the open source xash3d engine.

What’s the benefit of doing that please?

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.

Most engine reworks like that (Doom, Age of Empires, Heroes III, etc) offer significantly improved performance and scaling for modern computers.

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

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?

bypassing Steam I presume?

The offline system for steam is way less glitchy than it was pre-2010. Like another commenter said though, you may need to be online the first time you launch the game.

There is also the Goldberg Emulator [0] if you don't want to rely on Steam at all.

[0] https://mr_goldberg.gitlab.io/goldberg_emulator/