Dave Meltzer notes in this week's Wrestling Observer Newsletter that the belt's conception is due to the deeper decline in Raw's ratings, particularly how badly the third hour has been sinking in recent months.
USA Network has been pushing McMahon to do something about the third hour, which is part of the reason why the "Wild Card" rule is in effect, to shore up Raw's (and SmackDown's) overall star power.
According to Meltzer, USA Network had been sending various suggestions to WWE on how to increase the third hour ratings, and, according to a person familiar with the suggestions, they were all terrible. However, the 24/7 title idea was the best one, per the source, and McMahon knew he had to take one of the suggestions, so that's what they went with.
Though the title can be defended at any time, hence its name, the idea seems to be to emphasize defenses in the final hour, with the hopes of enticing fans to stick around.