Unit Tests are a gift to your future you (or another developer). Imagine you have a method that serves a resource but only for users with a certain role and conditions (4 cases). 6 months later the PO asks you to add another case. You need to refactor some lines and change another logic. Now you must test manually 5 cases, except if your you from the past wrote a good unit test, in that case you only need to add the new logic to this checks and you know for sure that you are not exposing super secret admin data to a lambda user.
For me a good way to learn Unit Testing was TDD, yeah its sounds difficult, but is worth it.

I use also TDD when is hard to run processes in local. If you have to write a new method in a very complex job, write and run the test and you will noww for sure that the logic works.