Why Mac OS X requires any amount of free disk space?
The amount of space the OS needs is neither fixed nor predictable. It depends on a huge number of factors: what programs you run, what you do in them, what background activity they trigger, what background activity happens for other reasons, etc. Hard drive space is used for overflow from RAM, logs, temporary files, etc.  If the system reserved a fixed amount of space for this, it would wind up being bigger than it needs to be a lot of the time (and hence waste space), and too small some of the time (which would be very bad).