Why cheesecake needs hard base?
A cheesecake doesn't need a hard base, or any base at all. The cheese part of cheesecake is sticky and will stick to a pan, although there's ways to get around that. A base (whether spongy or hard) also gives a texture and flavor contrast which many people find works well for them. 

It's just tradition really, you can do what you want with it, I've seen cheesecakes baked in individual ramekins with no base, it's really up to you.