That was so that

1) He pulled the curtain back. That was so that we could see what was going on in the room.

2) He pulled the curtain back. That was for us see what was going on in the room.

His intention was to enable us to see what was going on in the room.

=

3) This key is so that you can come and go as you wish.

4) This key is for you to come and go as you wish.

This key is supposed to enable you to come and go as you wish.

I don't think '3' works.

Gratefully,

Navi.