A13: Every time you would choose a Daily Attack Spell (level 5, 9, and so on) you choose two to add to your spellbook. The same goes for Utility Spells.
Then, when you prepare spells for the day, you would prepare one for each level you know (1 level 5, 1 level 9, and so on).
It says prepare the number that you can cast per day, but it also says that you can't prepare multiples of the same spell. So basically you prepare one of each level.
Also, when you reach level 15, you replace both of the level 1 Daily Attack Spells that you know with the two level 15 Daily Attack Spells that you want.
Essentially, it just gives wizards some options for versatility each day.
It's an excuse to give the wizard a spellbook, and it's a throwback to Vancian magic.
EDIT: It gets even better if you choose the Expanded Spellbook Feat...