The debugger is able to read and write the target process' virtual memory, and even adjust its register values through APIs provided by the OS. See the list of debugging functions for Windows.

What do debuggers do?

Debuggers allow users to halt the execution of the program, examine the values of variables, step execution of the program line by line, and set breakpoints on lines or specific functions that, when hit, will halt execution of the program at that spot.

Are debuggers worth it?

Debuggers are excellent tools and allow seeing the state of the code as it runs and spotting and fixing problems quickly. Having such tools and using them should not translate into bad coding practices.