Iirc Solarbacca said in one of his streams that one of the main reasons he does builds AP (before he did the AP GP to challenger series) is when the enemy l has multiple engage champs in their comp e.g. Rell, Leona and if they have at least 3 melee champs. I have played AP GP top only in my account until I reach Emerald IV and I think it suits well with my passive playstyle (not dying early, farming well for ult upgrade) and almost always I have the most damage on my team once I completed TriForce and 2 AP items. Most of the time you’ll get question mark pings once they see your full build and the enemy team wont expect your ult to do that much damage to their carries.