Do you mean the way the actual pickup is positioned (slanted) on the guitar (like the bridge pickup of a strat)?
If so, I think that originally it was to further exaggerate the brightness of the top strings.
I personally like the slant to be the other way around where the pickup's high string polepieces are more towards the neck and the low pole pieces are more towards the bridge.
This warms up the thinner strings and "un-muddies" the lower ones.
I'm thinking that Mac is right. The reason those pups are angled is for tone, most likely to compensate for the brightness of a single coil in a strat. When its slanted like that it makes the top strings a little brighter because they're dark as it is.
AFAIK, that's exactly why it's done - for example, the same pup in the neck or bridge pos on a guitar will sound different - closer to the bridge is more trebly & brighter, closer to the neck is darker, warmer.