Replaceable things aren't essential — they can be replaced. The ink cartridge in your printer is replaceable, but your best friend is not.
When you think about it, most objects are replaceable, whether you're talking about the tires on a car or your television or a ballpoint pen. All of these things can be replaced with new versions of them once they break, wear out, or stop working. People are not replaceable — in fact, you could describe them as irreplaceable. Both words come from replace, with its "again" prefix re- and place, "to put somewhere."