I don't carry music, and I don't carry videos, but my laptop always runs out of space because of all the satellite data I try to keep there, for processing and testing.
These data sets are huge. Laptop computer is a bit big and heavy to carry around, and so I rely a lot on my Nokia N800 these days.
There's no reason I can't process data on the device itself (particularly the improved Pandora), but just to have the files available in a usable manner (listing tools, inspection tools, even just the file manager) is good.
And I also have tons of documentation in the form of pdf files.
The gigabytes add up fast, so I can in practice never get large enough SD cards. And the two card slots is a life saver, when you think about it - not just because of the double capacity.