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In the good old days, the BIOS did more than just handle the boot process, but it also contained the hardware interface codes. So if you wanted to scan what keys were pressed on the keyboard you would call the appropriate BIOS routine, if you wanted to write a block of data to disk you would call the appropriate BIOS routine - basically the BIOS was an early device driver.
These days if you are running Windows 2000 or later, or Linux, the OS device drivers will be talking directly to the hardware and the BIOS is largely redundant. However, if you are still running MSDos or the Dos based Windows (95/98/ME) then the BIOS will be being used - and although I suspect few would be running MSDos on a newly purchased machine on a regularly basis, there are plenty of recovery or firmware update routines which do boot into Dos or a Dos variant.
The amount of space that BIOS takes up is minimal (little more than a few MB, possible even only a few hundred KB), so it isn't worth worrying about given today's memory sizes. However, the systems also needs to reserve a block of memory for memory accessed hardware - a modern graphics card may reserve 64MB or more for its purposes! This can be a problem for 32bit OS's since that reservation has to be below 4GB (which means that the maximum available RAM for a 32bit OS is about 3.2GB give or take a few MB), and any RAM installed above that goes unused. On a 64bit OS, you can map this reserved RAM to well beyond the physical RAM in the machine (well, at least until we start seeing machines with 16 Exabytes of RAM installed).