If my FMC fails I lose my map-display, but the VOR/NDB needles and DME readings are still there.
If my LCD display fails I lose everything but my standby instruments. And these have VOR/NDB needles incorporated but no GPS info.
And of course the GPS receiver itself or the antenna can fail.
On small aircraft I had the GPS display freeze for more than 20 minutes on an IFR approved Garmin.
And why use IRS instead of GPS directly when the former needs to be corrected by GPS regularly??
One reason is because the IRS can do things that the GPS doesn't. For example it is one of the sources for the IVSI, flight path vectors, trend vectors, wind calculations, wind shear detection etc. In modern aircrafts it is also the system that provides the displays with attitude information.