Here is why you need to use the log chart:
(d/dx) log x = dx / x
Now, what is dx / x. Well that is the change in x over x. If we move from continuous time (derivatives) to a discrete time interpretation, then it's analogous to (x2 - x1) / x1 (ie. percentage change). Log charts show percentage change.
Why percentage change?
Well, would you rather have your investment go from 5 to 10 or from 10 to 20? Obviously they are both the same, you double your money. A log chart would show a change from 5 to 10 to 20 as a straight line which is what you'd want. A linear chart would make it look as if they price had grown exponentially.