Because the clutch plates are a wear item. Every time you pull in and release the clutch the clutch plates have a little bit of material wear away. Over time the clutch plates become thinner and will eventually slip whether the cultch is pulled in or not. At first you won't notice anything but eventually you'll start to notice that your launches aren't as crisp and slipping on acceleration under loas and/or trying to accelerate at highway speeds. All that unnoticed slipping will cause premature clutch wear and failure. Adjusting the clutch pack keeps spring pressure at optimal levels and prevents premature wear.
That's why.

 

hey if it works great then leave it alone.

 

that's what i do. just because the book says to adjust the clutch doesn't mean it needs it. if i like how it's working i'll leave it alone, if i think it needs to be adjusted i'll do that too and it doesn't matter what the mileage reads.

 

The problem is that by the time you feel it slip and think "I should adjust the clutch" it's already done damage and you've hurried along the time to replace the clutch pack.
Personally, I use the friction zone pretty extensively and readjust whenever I feel it move out of where it's comfortable to use. After the first time or two it only takes 30ish minutes to do.

 

There's the Lone Star Rally in a couple of days. I'll defer any clutch adjustments until after that, particularly since it feels perfect for the time being. We have several weeks left of riding weather left, but when it's over I'm attempting my first big bore. It'll probably get a new clutch then, or soon thereafter.

 

If your clutch feels fine, do as you wish.
For me, at oil change time, I go ahead and adjust since it's all open and right there in front of me. A little check that I do is when I loose the nut I go one round out, screw the shaft all the way in, and count the total rounds to get a general idea about wear. Then I go about my usual adjustment, half a round and lock it down.
It may not be exact, but it gives me some indication.