They are there to keep the fuel line from laying directly against the hot head. I have run with and without them and have not noticed a difference one way or the other.
They are simply heat shields for your fuel lines. If your engine does not get hot enough to melt your fuel lines or overheat your fuel, then you do not need them.
Being as the fuel line WRAPS around the head in most cases it would be a safe bet to use them. You can buy them for about 2.50 for the pair. Especially when you hit a lean condition thats unforeseen. Could be a mess otherwise.