This answer is long after the fact, but since I found this on google maybe this will still help someone. I just want to clarify what JasonD and Notabene were saying: It is a lot easier to do clipping calculations (figure out what you should see and what you shouldn't see because of which way you are looking, how far away it is, ect.). Instead of checking if things intersect the planes on the borders of your view frustum, you simply compare the x,y,z of everything to xMax, xMin, yMax, ect. , since you simply have a cube.
It is a little more complicated if you want to only have part of something showing, but the math is still better with a unit cube than with a frustum.
A couple things I found misleading in other answers:
-You aren't shearing the sides off of the view frustum, you are sort of warping it into a cube using homogeneous matrix transformations.
-We aren't converting to a 2D screen with this step.
This step isn't necessary to do so.
Theoretically we could do all our work without converting the frustum to a cube first, which would be more intuitive but harder math - but graphics is all about doing calculations really fast since there are a LOT of calculations per second for the average game/whatever.
It isn't necessarily a unit cube we are converting to, it just has to be a
rectangular box for our max-min calculations to work out.
In fact in class we used a box where the camera faces down the z-axis, z goes from 0 to 1, x goes from -1 to 1, and y goes from -1 to 1. In general in math 1, 0, and -1 are good numbers for making calculations easier, I assume that is why we don't go from -100 to 100 or something.
It makes clipping easier.
Edit: bobobobo has the gist of it. Everything is triangles, generally :D.
Source: Taking a university graphics class