PAT (Portable Appliance Testing) is carried out to ensure equipment plugged into a mains supply is electrically and mechanically in a sound condition.
There is a visual element, and a combined visual and electrical testing to complete the full test.
The actual requirement for needing a Portable Appliance Test is a bit of a grey area.
There is no real legal requirement to have the test specifically carried out.
There is however many parts of the Health and Safety at work act, provision of use of work equipment act and other safety documents which make reference to equipment being in a safe operating condition and regularly maintained.
Some venue insurances or health and safety procedures may also request a Portable appliance certificate as a standard working document. The Code of Practice of in service electrical equipment is a guide and procedures document issued to ensure all testing is carried out to advised parameters and clearly indicates compliances of passing and failing.
Is it mandatory to be tested every 12 months? The answer in short is no, however there are recommended minimum retest periods for testing based on risk assessments.
If your equipment is used everyday and is moved around in different venues then actually you may require more frequent testing.
Only generally offices have their test periods extended where a PC is not moved for several years it may not be subject to as much wear and tear as an item used by bands every day.