Most of the time the "sizing" isn't needed. But I've had chambering issues with cast loads prior to buying mine. So it's cheap insurance from that standpoint. Additionally the ability to quickly adjust your crimp without having to monkey with the setup on the seating die is very convenient, to me anyway.
“At the core of liberalism is the spoiled child – miserable, as all spoiled children are, unsatisfied, demanding, ill-disciplined, despotic and useless. Liberalism is a philosophy of sniveling brats.” — P.J. O’Rourke