- Can you give me $5 so I can take the bus to town? = so [that I am enabled to] take the bus to town? -
- Here is $20 so [that] you may take a taxi = so [that you are enabled to] take a taxi.
He kills people [in order to protect our way of life] so that you can [do a normal job, i.e. that you can, if you wish,] be a mailman [like you are.]