1
general::
1. In order that, as in I stopped so that you could catch up. 2. With the result or consequence that, as in Mail the package now so that it will arrive on time. 3. so … that. In such a way or to such an extent that, as in The line was so long that I could scarcely find the end of it. All three usages date from A.D. 1000 or earlier, and the first two are sometimes put simply as so, as in I stopped so you could catch up, or Mail it now so it will arrive on time.
American Heritage Idioms