Because, quite honestly, if you are not tracking and testing your metrics, you are leaving mountains of cash on the table.
If you have a conversion rate of 1% and you boost it to 2%... guess what? That's doubling your income!
It just seems that most of the discussion regarding tracking/testing is in the paid traffic world.
But if you get ANY traffic from anywhere, you need track it and test it!
There's better tracking software out there, but a great free and powerful tool is of course Google Analytics.
Its goal tracking features are pretty awesome for tracking conversions from 1 page to another.
And things you want to test for are copy, headlines, sales funnels, opt-in forms, etc. I think the best marketers are always tweaking these things.