Why are depth buffer values clamped to (0, 1)? This is a simple question.

I know that in OpenGL or other graphics API, the values stored in the depth buffer are automatically clamped to (0,1).

I just want to know why people do this instead of just keeping the actual depth values as float. Is that just because of the non-linear depth equation which makes the closer depth values more precise and the further depth values less precies?

Thank you!
(Just converting comment into an answer)
It seems you might be asking one of two possible questions.

If you meant "Why doesn't the Depth buffer contain camera space Z"
then it's perhaps worth reading Why do GPUs divide clip space Z by W, for position? 

If, OTOH, you just meant "why is it restricted to [0,1)?", then that's probably because old systems represented the Z-buffer depth with fixed point numbers, hence a natural [0,1) range.
 Of course, later hardware arrived that could use floating-point depths; IIRC Dreamcast's depth representation was an arbitrary positive float representing $\frac{1}{w}$.