Sham, the chief reasons to transer into a bucket have been mentioned - good stirring of the priming sugar, and getting beer into bottles with a minimum of trub disturbed. However, there are arguments FOR bottling from the carboy. The chief one is that there is a layer of CO2 on top of your beer in the carboy, and you can basically add the priming sugar, and stir the thing, without ANY exposure to oxygen, unlike when you transfer into a bucket. Just gently funnel in the sugar solution, and stir gently, and let it sit for a half hour. Unlike others here, i believe that once sugar is stirred into solution, it stays there for quite awhile. But make sure you do give things an initial stir.
The trub is not much disturbed, and if you let things sit awhile, then it settles right back down. I've had no clarity issues bottling this way.
So... if you find it easier to bottle out of a bucket (with syphon or tap) then great, do that, and if you want to keep your beer in the carboy, then you can do that too.