The under seat tool lockers get wet no matter what. Unless you never drive in the rain or leave your truck outside that is.
The relays corrode some, and can fail over time. Its cheap insurance to carry one or 2 with you. If you have issues switch... them out, it just might get you back on the road without the need for a mechanic. In a lot of cases, with the 90s getting on in years now, they just fail.
If you carry a spare, and you don't hear your fuel pump when you turn the key, try changing the relay. You'll look like Macgyver to your lady when you fix it yourself on the side of the road.