If you plan on using a GPS approach, you are required to do a RAIM check as part of filing the flight plan. There are prediction sites for the airport as well as enroute.
It is especially important at certain locations along the equator and in mountainous area, especially in China and others.
RAIM prediction for enroute will predict how many sats will be available. In reality you need a minimum of 6 sats for navigation. So the prediction will let you know how many are estimated for the final approach and enroute.
Depending on your level required during flight, you may have RAIM failures and have to go to a higher level, or disco the procedure.
Have seen many flights NA into areas of China due to RAIM check on dispatch.