Much like Cordelia/Lindsey, this is a 'ship with little canon but strong fan support whose major website has unfortunately disappeared. However, unlike Cordelia/Lindsey, the parallels and reasons for Cordelia/Spike are more evident to the casual viewer and reader.
First of all, Spike and Cordelia are quite simply, the two classic "snarky" characters of Buffy. Spike appreciates good-looking women, and Cordelia would be more than happy to dole out a little discipline to the playful, wayward Spike. At least, this is the evident rationale. A great deal of Cordelia/Spike fic relies on the fact that Spike adores women with a sharp tongue, and that Cordelia's snappish side is more than willing to be dominating in the bedroom as well (see Cordeliafic Cliches 101). At the same time...are the fans wrong?
After all, Spuffy happened, and Cordelia and Buffy are strikingly similar in many ways. Furthermore, Drusilla and Cordelia have some physical similarities. Add to this the tendency to place Cordelia/Spike before Angel Season Three and Cordelia's great transformation into a less...snarky...character, and you have a set of reasons for Cordy/Spike that make legitimate fictive sense. Granted, Cordelia and Spike have had minimal screen time together, and Spike is the reason Cordelia took a rebar through the gut. Angel as Angelus was just as bad, if not worse, and Connor threw Angel into the ocean for three months. Spike could be forgiven.
So Cordelia and Spike...screwball comedy good fun waiting to happen...or perhaps canon distortion taken too far. Take your pick.