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The different ways a tire can be unaligned
(sh.itjust.works)
sorry for bad alt text, I lack the terminology to describe this accurately
(i'm not a mechanic, i have no earthly idea if this is accurate. Don't sue me)
In addition to what others have said, it also affects camber in turns. Positive caster when turning gives the outside wheel negative camber and the inside wheel positive camber to keep a larger patch of contact when turning at high speed.