A common issue I see for people introducing automated testing is estimating the testing and implementation of a task separately.
Something like, "It'll take x to do the work and x to write the tests for it".
As well as implying you'd write all the code and then all the tests or vice versa, if you provide separate estimates, it's easy for someone to think the tests are optional and can be removed.
If you provide one estimate for both, this can't happen.
