Mid at Top = Top vs Mid
Top builds more at Armor at start, so you have a better chance as a Ap Carry
Adc,Sup at Mid = Mid vs Bot
Mid builds more at Mresi or Ap at start, so you have a better chance as a Adc with a real support at your side
The Supporter can ward both sides and can choose if he wants to play deff or off against the Jungler
Jungler vs Jungler
But more chance at Counterjungling thanks coorp with the Support
Top at Bot = Bot vs Off/Tank
Top can play more at Tank if he didnt are sure against two guys.
Mostly you can play how you like but not to much carry.
But in the most games the Top goes at Bot, too.
Top + Bot just needing to ward 1-2 sides.