Because the enemy team has four AP.
You think to yourself "well... I don't think there are any better magic resist items so I guess I have to start building health and armor now instead"
WRONG
DO IT
that's a lot of lockets, you know you can have a max of 6 items, right?
Ok I equipped the Perfect Ascension summoner icon. Now what?
> if they have 4 ap. Now they cant kill a minion and you still give the aura :P
Most magic resist in the game.
PRAISE THE SUN
But the aura doesn't stack if your support got that item... (the actives are still useful, but the auras aren't)
Also, against 4x AP, 
and either 
or .
Replace warmogs with 
if you are ap and need an offensive item. If not jungling, it can be beneficial to get either a 
or 
in place of the cinderhulk item.
Love that item on tanks. Much easier to protect squishies and bruisers.
You still need armor, though. Towers, minions, monsters, and basic attacks are all still physical.
As a general rule, I try not to ever build all of one type of resistance. Odds are, they have SOME form of physical/magic damage that'll screw you in the end. Learned that the hard way when I built full armor, and Gangplank's ult would actually chunk me pretty hard (Had all armor+health items+cinderhulk and glory).
are usefull to, and give a tank "Ganking/pushing Power" in other lanes as well.
Even With 4 AP, I don't know if I would go both and . I would probably pick which one suits me better a some , but I probably would have started with the .
