The iPad can only draw more than the 20W when it's below 60% charge or so. Then it can draw around 25-29W if I remember correctly. A faster charger will help to get it back up to 50% a couple minutes quicker.
But it has another more significant advantage: If you want to keep using the iPad whilst charging it also helps with the charge time since it can both power the hardware and keep charging the battery at or near full speed. Which the 20W adapter can't do at all. You're looking at 4 hours or so if you use the iPad whilst charging, and that is way too long in my opinion.
I fast charge my iPads because the total charge time of at least 3 hours, more when in use, it's way too long. I have a tiny Anker 60W charger that can supply as much as the iPad can possibly draw, although I am not sure if it can draw more than 30W at all.
It still charges rather fast when the battery is near empty and I am running intense workloads with full brightness (though the iPad gets really, really warm, almost hot). My total charging time ends up at around 1-1.5 hours. I don't charge to the full 100%, these last few steps always take long so I take it off the charger at around 85% and don't bother with the rest.