The first Telecasters had their bridge pickups angled to exaggerate the deeper tones of the low strings and the brighter tones of the high strings, to emulate the tone of a lap steel guitar. The bridge pickup of a Stratocaster was angled too to, in theory, give you the same tone as the Telecaster, though in practice Strat pickups are too different from Tele pickups to copy the tone.
Even when lap steels went out of fashion, by that point people were so used to Telecasters and Stratocasters having their bridge pickups angled that they just left it like that and now it is just accepted that that is how all Strats and Teles should sound. It wasn't really until the mid-70s that people started to accept the idea that guitars didn't all have to be made exactly the same as the original 1950s designs. Even today, lots of people only want 'vintage' style guitars, so the angled bridge pickup lives on.

In the late 70s some people began putting their humbucker pickups at an angle, too. This was more due to more parts being available and different manufacturers making bridges with different string spreads to how other companies were making their pickups' spread; angling the pickup helps make the poles line up under the strings more precisely. In practice this doesn't actually matter much, so people stopped doing it other than a few 80s companies and people who wanted to copy Eddie Van Halen. Angling a humbucker doesn't affect the tone as much as it does with single coil pickups.

You might sometimes see people angle their middle or neck pickups, too. With single coil pickups this is usually to get a brighter sound. With humbuckers it is usually just because the person thinks it looks better.

So, it's a mix of tone and tradition.
Yes, I know everything. No, I can't play worth a damn.
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