[daenz]

I came at this from the perspective of the Binomial theorem:

(x + y)^n = sum{k=0,n} (n choose k) x^(n-k) y^k

And since a powerset is:

sum{k=0,n} (n choose k)

The way we can turn the first equation into a powerset is by setting x=1 and y=1, hence:

2^n = sum{k=0,n} (n choose k)