Test Expense

Once an automatic test is written, it can be run by a computer at the cost of a few joules. The equivalent manual test requires a person on payroll working down a list of instructions.

Test Reliability

The computer can be trusted to faithfully execute the same test procedure, every time. The human is apt to make mistakes and get lazy.

The computer's testing failure modes are also much more readily apparent - it crashed (test reports stop appearing), it had a bit error that caused a false test result (run a deterministic test again, and the result differs). If a human misses a step and checks off the "OK", how can we tell?

Test Durability

An automated test has to be a concrete artifact (e.g. a piece of code) in order to run, and is naturally included with the other software development artifacts - the source repository. A manual test may be developed on a sheet of note paper by a tester, and never formalized. The business is more likely to need processes in place to ensure that doesn't happen.

Test Value

The computer can be programmed to output test results in a consistent, easily analyzed form. The person is either doing data entry to generate the same, or is recording free-form notes that require an analyst, developer, or manager to digest.