The -3db point is also known as the "half power" point. In voltage it may not make not make tons of sense as to why we use (sqrt(2) / 2), but lets look at an example of what it means in the sense of power.

First off, power=V^2/R, but lets assume R is a constant 1ohm. Because of the constant 1ohm, we can remove it from the equation all together.

Lets say you have a signal at 6v, its power would then be 6v^2=36W.

Now I take the -3db point, 6v*(sqrt(2) / 2) = 4.2426V.

Now lets get the power at the -3db point, 4.2426V^2=18W.

So originally we had 36W, now we have 18W (which of course is half of 36W).