One reason I can come up with is as far as my company is concerened, is that if, for example, a customer clicks on an email, and maybe then attends a trade show, but goes dark for a year. Then, after when a sales rep stops in to check in/cold call, they decide to move forward to a quote (deal). Our reps are instructed to use the most recent point of contact as the lead source, so if the deal was automatically attributed to the email, the reporting (for us) wouldn’t be accurate. If I’m making any sense… 
In our world, we’ve come up with 12 possible lead sources.