The short answer is repeatability. We automate a test because we need to execute the same tests over and over again. Would you want to automate a test if you were only going to run it once and forget about it? Of course not! The time and effort that you spend on automating the test, you could have executed it manually.
Now, by definition, we automate repeatable tests, i.e. regression tests, that we need to execute frequently.
So, next time, when you want to automate a test, take a step back and think how often are you likely to execute this test? Is it really worth the effort to automate the test?