As alm asked, where did you read that the average DC value is important? If you have a source that says it is important, yet it doesn't tell you
why it is important, then perhaps it isn't that important after all?
When it comes to averages, average voltage is like average anything. You add up all the values over a certain time period and then divide by the number of samples. Or if you have the voltage as a graph you can add up the area under the graph and divide by the time interval.
Mathematically the average looks like this:
This is how you write "the area under the graph divided by the time interval" algebraically. A simple voltmeter will do something like this.
However, sometimes it is useful to consider the power delivered to a resistive load. There is a different average voltage, the RMS average, that would give the same power as the voltage being measured. To calculate it, we can see that the power into a load is given by the following formula:
W is the power,
V is the voltage and
R is the resistance.
Now if we look at the total power over a period of time, we get this equation:
This is now a different average voltage, one which will produce equal power to the true voltage. If we rearrange this equation to find the average voltage, and call the average voltage the RMS voltage, we get:
This is called the root-mean-square average, for reasons indicated by the equation.