Actually I am from Mainland China and the access to the Vanilla Launcher is unavailable (due to its high price and inaccessiblilty of the Internet). 3rd-party Launcher like HMCL, PCL are very popular among Players in Mainland.
First of all, adding mods is fairly convenient on HMCL ---- it has a specific function for you to download, to add, to update, to delete, automatically. As for the Vanilla, I have to afford the lag to download a Forge client or a Fabric one, and configure them MANUALLY.
Then, they allow you to log in as an offline user, merely preventing you from using the Realms and joining servers that requires a online account, for free. It really allows many players who adore Minecraft yet can't afford the cost to get a Vanilla Launcher to enjoy the game.
Though many people around me think them invalid, they outperform the Launcher that Netease develops (the story of Minecraft and Mainland China is too complex to be written in this post, so if you are interested you can subscribe to me and I'll narrate the bumpy history)
