I am no expert either, but my main purpose for treesitter is better syntax highlighting although it does other things as well. I think it is pretty common to use LSP and treesitter together e.g. Kickstart.nvim does it.

Not sure how this relates to autocomplete. For that you usually have things like blink.cmp, coc.nvim or nvim-cmp. I think they mostly work together with an LSP, but maybe they can use the syntax tree from treesitter for that as well.

Neovim also has built-in treesitter support. The plugin just made handling the highlighting parsers easier.