cargo add <dep>
is a relatively new command. For a long time you had to edit the Cargo.toml file to add a new dependency. So a lot of tutorials still use that as a way of adding a dependency. And other guides often copy from the older ones. They also can describe it this way as a way to introduce the Cargo.toml structure since it is not hard to hand edit. cargo add
is just a convenience after all and it is still worth understanding the Cargo.toml structure.
But yes, many people do use cargo add
for simply adding deps rather than editing the toml file. Though it is not uncommon to edit it by hand either.
rust-analyzer I believe will compile all the deps the first time it runs. This will take some time but after that it should be fast. If you run a cargo clean that will wipe everything and you will have to wait again. So dont do that. clean should be something you run sparingly not before every compile.