Mixed foursomes uses two sets of tees. Compare that to just straight mens foursomes where everyone is always playing each hole from the same place. In mixed foursomes you could actually play a different length of course to the rest if the field if you allowed the partners to choose
That is a significant part of the strategy in mixed foursomes.
It is also an option in mens' foursomes if each player is permitted to choose which rated course he will play (with the appropriate handicap adjustment).