Why ballistics is important

From the Internets:
"It still mystifies me that Jack O'Connor and Elmer Keith killed anything. ...(blah blah)...
Sighting your 30-06, 270, 7 Remington or 300 Winchester 2.5" high at 100 yards will let you hold on hair way out to 400 yards which, frankly, if further than most shooters have any business shooting at game." 

 Here we have some actual data. You are off by 20 or 30 inches at 400 yards, depending on the load of course. Unless you are being attacked by a herd of dinosaurs, this is nowhere good enough way out to 400 yards.
If you spend $500 on your rifle, spend 5 minutes finding out what the drop is at the ranges you will be shooting at, and stick the numbers on your rifle sling.