It's a common practice now in many MMOs and even in some Final Fantasy titles like Record Keeper. That would motivate many players who play only 1-2 times per week to login (and play) more often, with consecuitive logins offering greater rewards.
It could be anything random, from a small gil or tomestone amount to a rare potion which instantly gives 1/2 or 1 level on any job it's used. And some legendary rare items can be exclusive only to daliy logins, like minions or glamour sets to make the feature even more appealing.