Consider the argument:

If p, then q.	If you play with fire, you will get burned.
p.	You played with fire.
Therefore, q.	Therefore, you got burned.

Any argument of this form is valid, because it merely unfolds what is implicit in the meaning of the hypothetical premise. That premise says that the truth of p would be sufficient for the truth of q. If we then assume that p is true, we may conclude that q is true as well.