Microwave leak detectors are a specialised instrument designed for one specific purpose. They do not have any other functions.
They are primarily used to check microwave ovens for radiation leaks. However, they can be used on other household items which emit microwaves at the stated frequency range, like mobile phones.
Although not necessarily a professional tool, they can be used by electrical PAT testers. PAT (portable appliance testing) is a part of health and safety regulations to ensure electrical items are working correctly.
A microwave radiation test is not a necessity when PAT testing, but can sometimes be added for an additional charge.