One really good reason is that vector is infinitely scaleable.
Psd is linited to your resolution at the point of creation.
We have all cursed at the logo that is 80 x 80 pixels that goes to a print ad.
Add gradients to something like that and you have to recreate it. Then you have to identify the font. Then you have to work from RGB to CMYK or PMS.
Hope this helps.
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