Okay, so it's really just a top-seven. I hope you don't feel cheated. If you do, then go here and read more about via-in pad.
Now, here are the top-seven reasons for putting vias in pads:
#7. Grounding the center flag pad on a QFN. All those little electrons might need a quick and easy way home - (or is it the holes that need a quick and easy way to go to work??)
#6. Cooling the QFN center flag pad. If light can be both a wave or a particle, why not heat? All the little Kelvonic particles will run down the via and hide on the other side of the PCB. Kelvons are better than Photons because Kelvons have feet.
#5. Greater BGA routing flexibility. Put the via in the BGA pad (make sure your board house fills it and plates over it) and you can run traces between the pads. Then your signals can have races on the traces.
#4. Keep all that messy solder off the top side of your board. It's easier to inspect a pcb assembly, especially underneath a BGA, if all the solder gets sucked down through a via to the other side of the board. As long as the circuit doesn't actually need to work...
#3. Promote tombstoning with small passive parts. Put a via in one pad on a small passive and take bets on whether the part will pop up like a tombstone in the other pad. Wyatt Earp can be PN# 478-1051-1-ND and Ike Clanton can be PN# 478-1055-1-ND. No unauthorized component substitutions, please.
#2. Presents for friends and family of PCB fab company executives. Doing it right by filling and plating over costs money. As we say in the marketing bizz, that service is a "high-margin accessory". Of course, doing it wrong, costs even more money.
... and ...
The number one reason to put vias in pads is:
#1. Get revenge on your manufacturing folks for all of those annoying documentation requirements they keep throwing at you.
Via la Screaming Circuits!