so vs. so that

*He wasn't dressed warmly enough, so that he caught cold.

That added to so changes the meaning from result to purpose:

He wasn't dressed warmly enough, so he caught cold. (result)
He dressed warmly so that he wouldn't catch cold. (purpose)

One source of confusion is that that is often omitted, although the intended meaning is so that (purpose):

He dressed warmly so he wouldn't catch cold. (purpose)

This is to be distinguished from:

He dressed warmly, so he didn't catch cold. (result)

The pause and comma are more likely before so (result).