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GR: why do particles follow geodesics?

In one of my gravity classes, we derived the geodesic equation for a curve xa(λ)x^a(\lambda) by considering the action

S=12∫gabdxadλdxbdλdλ\displaystyle S=\frac{1}{2} \int g_{ab}\frac{dx^a}{d\lambda} \frac{dx^b}{d\lambda}d \lambda.

We then use the principle of least action to compute the Euler-Lagrange equations, which are equivalent to the geodesic equation

ub∇bua=0u^b \nabla_b u^a=0.

However, when I questioned why we took the action to be what we did, I was told that we chose it precisely because it gives the geodesic equation that we want when we employ variational principles. Isn't this circular? Why is it that we care about geodesics in particular? Is this just a postulate of GR? If so, and if it is the geodesic equation that we are interested in - and we know that this is the case a priori - why do we need to bother constructing the action at all?

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