Because if you don't document where you found the information, you will do what I have done on more than one occasion:
Ask yourself - WHERE DID I GET THIS INFORMATION???.
And you will spend more time than you need to trying to find where you found the information in the first place. And with busy lives and sometimes not as much time as we would like trying to spend it on family research - why waste time?
Now, even if I do not have time to write the specific citation the way it should be written, I at least document in my genealogy database the source of the material and if time is short - I write down the source and the word INCOMPLETE.
Then at the end of every month, I make time to go back to the INCOMPLETE citations and complete them. This is not ideal since it would be better to document the sources and citations at the time of "discovery", but it works for me.