One mistyped parameter and a campaign shows up as two lines in your report.

They tell your analytics package where a visitor came from: which channel, which campaign, which ad. Without tagging, traffic disappears into the ‘other’ category and you know nothing.
Analytics packages treat Facebook and facebook as two sources. Your
report then shows two lines you have to add up by hand — and your averages are no longer
right. Putting everything in lower case solves that.
If you build a link yourself for each campaign, sooner or later you'll type something different. A template in a shared document or a generator takes ten minutes and prevents months of messy data.
A question or project on this topic?