It’s a good question, and I wondered for a long time as well. Then I discovered why, and it's actually pretty simple. It’s because of the Pythagorean Theorem. You know,
I discovered it while I was building a long truss, much like this one.
Remember that the Pythagorean Theorem only works for right triangles, so the formula works out to be
3^2 + 4^2 = 5^2
9 + 16 = 25
I know that once I figured this out, life got much better and I began to use these oddball beams a whole lot more often. The 3-4-5 right triangle is very useful in engineering and especially useful in building with Lego. I have used it several times, and I hope to share some constructions with you soon.