When you download something (e.g. view a website) you need to first request the information (i.e. Upload the address) then acknowledge the receipt of the page/file. All this is part of your upload allowance/speed.
This would be most peoples use for upload, but if you serve your own website/ftp server, or use P2P, then the speed with which you can send stuff to other people is limited by your upload speed
HTH
André