Why must contribute to CPF?

Silly me, why am I asking such a simple question? You need to build up a savings to buy affordable HDB flat mah. Tiok boh? Like that also dunno. You also need to have some money for retirement right? Tiok. And you need money to pay for big and affordable medical bills when you are old lah. So simple and I have to ask. And there are other things to pay for with the CPF savings, like Medisave