This isn't actually the case, though it seems intuitively so. As long as the descent rate is constant, this formula holds true.

While flying straight and level, lift must equal weight. When banked our total lift must increase to keep vertical lift equal to weight, just like when straight and level. But wait, doesn't lift also equal weight in a steady climb or descent?

In fact it doesn't, but the difference is so minuscule (<10 pounds in a Cessna Vy climb) that it can be assumed negligible. In other words, in a Vy climb less than 10 pounds of the aircrafts weight is being held aloft by the engine thrust. The rest is by lift, thus lift and weight can be assumed equal for any typical steady flight condition.

We conclude that a steady climb or descent (glide) must also have lift equal to weight. Thus, the formula 1 / cosine bank angle holds true because our goal is still the same as level flight -- get vertical lift to equal weight.