ATR is an excellent indicator for comparing volatility now and before. But what if you want to compare the volatility of two pairs?
Since each pair with different quote currency has different pip value, the output of the ATR doesn't really say that much.
Comparing pairs to other instruments is even harder.
What you as a trader care about is how much the pair has moved up or down in value, therefor normalizing ATR to percent would make things much easier.
True Volatility is really just the average true range indicator modified to put out average true range in percent instead. It also got some important levels pre plotted out for easier comparison.