Strictly speaking, this isn't a rune as such.
It came into being originally as a blank spare, much as the jokers in a pack of playing cards, so if one was lost, then a replacement could be made once, before the whole set had to be replaced.
As with the joker cards, the blank rune gained a meaning and value of its own, as below.
Some people will not work with the blank rune, others will want to. My set does contain a blank rune, though it rarely comes up in readings. If I wanted a set custom making now I probably would not include it, with the set I have, leaving it out seems to make the set incomplete.
Again it is personal preference as to wether you work with it or not.