Also, if you've inherited code, or didn't work on it for couple weeks, chances are you won't know/remember how to do the manual tests, and will waste time trying to rederive it from first principles. This also applies to any other kind of "ad-hoc" ops, and multiple times more so if the activity involves multiple runtimes - e.g. issuing a shell command. I've learned this the hard way, and these days I note down the precise steps/commands for any manual op that's not very clearly a one-time thing / throwaway. Your future self and whoever inherits your code will both thank you.