The act of disassembling is a manual one. You read the code and follow the possible execution paths within it manually. To an extent, you might be able to follow used labels like links, but that is still merely exercising the interactive functionality of the disassembler.
Debugging the code you are interested in provides a completely different viewpoint, allowing you to see what the code actually does, the values passed around in the registers or on the stack, with the application running and potentially interactive.
Debugging - Wikipedia entry for debugging.