Apart from being the law, it's a good safeguard, like wearing a seatbelt and buying car insurance.  If someone trips over your guitar cable at a wedding and hurts themselves, or if you plug your amp into the wall socket and cause electrical damage at the hotel, or if you knock over something expensive such as an antique ornament, or if you fall down some stairs and take a guest with you in the fall,  - then you are likely to get into trouble and could be sued by the venue, the guest(s), the customer, or all three, depending on the circumstances. 

This is not scare-mongering, it actually happens!  Please do not expect your application to be approved without proof of P.A.T and P.L.I.

It's good to send the Agency copies of your PLI so that we can vouch for you if ever a new customer asks. It gives them added confidence in you as a professional artist.