A parser turns its input (often text in form of a file) into a more advantageous representation (usually a certain data structure in memory) to perform a specific task.

Common examples include:

parsers for markup languages (e.g. HTML/Markdown etc)

parsers for data-serialization formats (e.g. JSON/XML/YAML etc)

parsers used as part of/in conjunction with a compiler (in this case, the parser consumes the tokenized input emitted by the Lexer and ultimately feeds the resulting parse tree/concrete syntax tree into the compiler)