Duty free receipts stapled in my passport

I bought a bunch of duty free souvenirs at Laox Akihabara. They stapled the receipts in my passport.

Do I need to keep the receipts?

That just depends how honest you want to be. They staple them there for customs declarations.

http://www.customs.go.jp/english/summary/passenger.htm

Purchase of Tax-Free Articles

When you purchase articles at specified "tax-free" shops, consumption tax on those articles may be exempted on condition that they are taken out of Japan. When you purchase tax-free articles, you will be required to present your passport at the shop and you will receive a card entitled " Record of Purchase of Consumption-Tax-Exempt for Export, " which will be attached to your passport. Articles purchased tax free will be checked with the card by a Customs officer, who will collect the card at the time of your departure from Japan.