Also useful for learning Mathematica itself.
For example you want to know how to integrate a function in Mathematica, then you enter
integrate x y dx
on Wolfram|Alpha and click on the result to see that the corresponding Mathematica plaintext input is
Integrate[x y, x]
Suppose you want to know how to graph a list of data points in Mathematica, you could enter
graph n^2 from 1 to 10
on Wolfram|Alpha and click on the 'Mathematica form' link (in the first box) to see that
Plot[n^2, {n, 1, 10}]
is the corresponding Mathematica form.
I wish other programming languages also have this feature.