Recently I've been seeing the power LED on my fender amp cut out. It ended up burning out and not lighting at all. This is frustrating if there are issues on stage during a gig as there is no way of telling if there is power going to your amp.
These replacement bulbs are cheap, easy to install and are very durable. If you have a fender amp that uses these, I highly recommend having a few spare in case of burn out. Although it's not going to affect the amps functionality, it's still important to know if you have power supplied to your amp for personal safety as well as equipment safety and care.