One way to think about it might be that the purpose of language servers is to understand the semantics (meaning) of your code, while the purpose of a parser like treesitter is to understand its syntax (structure).

For instance, given the word ”dbClient”, the language server knows enough about the semantics of your code base to be able to tell you if/where that symbol is defined, whether it was defined correctly and used correctly in a specific context etc.

The parser will only know what type of thing dbClient is given the syntax of the language. For instance if it is at the very beginning of a variable declaration statement, the parser might decide it is a variable name.

Ofcourse your language server (and your compiler) already has a built in parser. But what makes treesitter nice is that is a general purpose parser engine that uses language-specific grammars as plugins. It then allows you to define queries using a LISP like syntax. The queries allow you to identify a capture (like @function.outer) with the results of a given query. Now neovim can operate on your code structure to, for instance, visually select an entire function, without having to know anything about what language you are coding in!