Well, ease of use I suppose. Just look at us capable of using basically anything as an energy source. Imagine a machine that is not finicky on how they got their energy. The problem for any such machine will always be efficiency. You cannot do more work than the energy that you put in a system. Such machines will need a stomach that is capable of processing “food” at the same/greater speed that it is spending its energy. Not to mention we usually use machine for heavy, energy intensive task, so I doubt such stomach will be useful for any meaningful machine.