Applying for multiple cards in one day means that companies don't see any of those card inquiries on your report when they're pulling your score/report to review for approval (though it's usually automated these days for most banks).

The advantages of this are more frequent immediate approvals, whereas waiting a week in between apps could raise a flag to the 3rd and 4th bank you apply to that you're credit hungry and either deny outright, or force a recon line phone call, since they would be able to see the inquiries you did in the prior few days.