The difference from taking the advance vs waiting for the backend money is whether or not you're stable or comfortable enough to wait for the royalties to start rolling in which will take some months/years.
If you broke and you got sht to do and pay you go be like fck that waiting for months and years sht pay me 2 mill now.
If you already got bread or if you just stable enough and you can see the big picture you'll be like ok I got enough to hold me over until the first check come so I can wait these months or years out.
He must've already had money or his mama or family did.