As I've mentioned elsewhere when the subject comes up, Canon liked to introduce an important new camera with a little brother camera that a couple of the advanced features removed, but at a lower price. I'm not sure if it was to appeal to conservative photographers who didn't want a lot of new features bundled together, or whether it was to appeal to people who couldn't quite afford the better camera.
Doesn't matter, they did it several times, such as with the Canonflex RM and RP (introduced on-board camera metering), the FT and TL (TTL metering), the AE-1 and AT-1 (shutter-preferred automatic exposure), and their EOS 620 and 650 (autofocus). They also did it with the Canon FX and FP: these introduced Canon's new bayonet FL lens mount. FL lenses normally have the diaphram wide open for focusing and composition, then you can quickly close the diaphram to meter the scene, and then go back wide open again. When you press the shutter, the camera closes the diaphram to where you set it, fires the shutter, and opens back up again.
That may sound like a hassle now, but at the time it was a great feature. Only having to briefly stop-down to meter was a convenience.
The only difference between the FX and the FP was the addition of an on-board CdS light meter: the FX had it an the FP did not. If you bought an FP, you either needed a hand-held light meter (which many people had) or used the Sunny 16 rule. Given the choice, I don't know why people wouldn't go for the FX, but I suppose if you were strapped for cash and you already had a Weston Master IV in your bag, why not?
If you bought a FP and decided you wanted an on-board meter after-all, you could still buy it as an accessory. This is the primary reason I ended up buying this FP, I had the meter and I wanted to marry them and show them here. I think it looks pretty slick. The first time I saw the FP with the meter attached, I thought it was an interchangable prism, which is what Nikon and Miranda were doing at the time. Accessory meters, even those built for a specific camera, typically just attached to the accessory-shoe the way the Yashica Penta-J does.
Not this one. It covers the pentaprism entirely so it looks like it's all one piece. Nice design. A shame they didn't do that with their Booster finder.
For more information: Canon Camera Museum
Camera manual: Orphan Cameras.com