Invitations

So we have picked out our invitations and was trying to decide between 2 different designs for our Save the Date. I was talking with my daughter earlier today and she asked me a question that now has me second guessing the invitations.

On the back of our Save the Date (STD) is the venue name, the address, the date of our wedding, and the time. She asked me "why are you getting invitations if that's already there?".

My friend recently got married and that's the way her STD was and it was super convenient for us to keep up with. When we got her invitation, we put it in a drawer and haven't looked at it since.

So my question is...do we need both if that information is on the STD? What's the proper etiquette for this? Any suggeations?