Application stability (interface)
Changing application interfaces (e.g. GUI) can impact the automated test ware. This may force the update of automation interfaces (object repository, scripts, ...).
Changing functionality can impact the automated test ware. Composition of tests may need to be altered/updated.
Good quality manual tests
A poor manual test will remain a poor test, whether executed manually or automated.
Frequently run tests/functionality
Higher ROI when automating tests/functionality which require many runs.
It may be possible to reuse certain script libraries for different applications.
Limitation of user errors
Automating repetitive test procedures decreases the risk of user errors due to loss of concentration.
Complexity of application/tests
Complex applications may require extensive training of test executors.