What is the purpose of putting ad/support top?

The reason they do this is to deny Gold/XP to the enemy's Solo Top and AD Carry, getting an advantage vs certain hard team compositions.

For example: Versus an AD Carry Graves+Alistar/Solo Top Talon. Swapping lanes makes the enemy Solo Top and AD Carry lose allot of CS and XP and maybe confusing them in the process, making them prone to mistakes that normally they wouldn't make.

You can do this with a Cho'gath solo top, I believe. Since he can regenerate his HP easily and farm well even vs 2 opponents due to he's AoE spells.

Check out this game CLG.NA Vs MiG.Blaze from the Korean Azubu LoL The Champions Spring 2012 tournament

Hope this helps :).

Support/AD is used when the opposing AD/Support is stronger/better skilled players than your own Support/AD. As a result, it allows your own AD/support to 2v1 a lane against a top solo which allows you to farm with ease. However, a strong bruiser with sustain has to be used for your own bot lane to survive 1v2 against enemy AD/support.

It's also used for shutting down their top solo at times if their top lane is extremely weak against AD/support 2v1 lane while your top lane is strong for 1v2 at bot lane.