An 'HD texture pack' is usually a higher-resolution texture pack, released optionally for people who have powerful enough hardware to upgrade the visuals. Unfortunately, most 'HD texture pack' DLCs made by developers aren't very good, and are usually outdone by modders. Skyrim is a good example.
Normally the texture pack will only include higher resolution textures, not models. And it shouldn't have a noticeable performance impact if you're not maxing out your VRAM. So if you have enough VRAM, it basically makes the game look better. Normally in modern games you see less than 5% performance difference between the lowest and the highest texture settings while not maxed in VRAM. Normally these high resolution DLCs include the information on how much VRAM you need to run it.
As for the reason it's available as a DLC: Often these high resolution textures are a big reason why games take so much storage space these days. Developers sometimes take the highest texture setting and make it available as a free DLC so the people that want to use it can still download it and the people who don't need it don't have to download all the extra stuff. For example the high resolution textures for Final Fantasy 15 are over 90GB and thus the developers made it availabe as a DLC. Another game that does it and i know if is Rainbow Six: Siege.
You can totally run the game with HD textures with a 1060 or similar but you *will* have to lower settings otherwise, which kinda defeats the purpose of higher resolution textures, for example with my 1060 I had to play with "prioritize framerate" which is basically dynamic resolution and looks rather muddy in order to achieve 1080/60 with the texture pack. Now with my new and shiny 1070 I can set it to "high" which is the highest setting for image quality while still maintaining 1080/60 and the image quality difference is quite stark to be honest. Highest measured VRAM *usage* so far was 7.5GB btw!
Announced at E3 2017, Monster Hunter: World adopts the series' standard formulas from its older home console roots and recent handheld games to take advantage of the higher processing power provided by modern consoles and computers. Changes made in Monster Hunter: World include creating environmental spaces that are fully connected and removing the "zones" that were necessary for the PlayStation 2 and handheld games, more advanced monster artificial intelligence and physics, a more persistent cooperative multiplayer experience, and a refinement of the game's tutorials and user interface to help with bringing new players into the series. These changes led Capcom to plan for the game's simultaneous release worldwide, since Monster Hunter as a series has generally languished outside of Japan partially due to disparate release schedules. Capcom also opted to support online play between these different geographic regions for similar reasons. The delay for the Windows release was attributed to Capcom seeking to make sure its first foray into the Windows market was optimized for players on computers. By April 2020, updates to the game were synchronized among all platforms.
Monster Hunter: World received critical acclaim upon release, with critics praising how Capcom was able to make the game more accessible to new players and to Western markets, without detracting from the series' core gameplay elements and enjoyable difficulty, and fully taking advantage of the computational capacity of modern consoles to create living ecosystems, with some even calling it the best in the franchise. Monster Hunter: World is the single highest-selling game in Capcom's history, with over 21 million copies shipped by July 2022. A DLC expansion pack, subtitled Iceborne, was released for home consoles in September 2019 and for Windows in January 2020, and reached over 9.2 million sales by May 2022. The next mainline installment, Monster Hunter Rise, was co-developed alongside World and announced for a worldwide release on Nintendo Switch in March 2021, with a Microsoft Windows version released the following year.