Letters, that is. Flip that p over the horizontal axis and you’ve got a b. Flip it over the vertical axis, then back over the horizontal, and you’ve got a q. A u is a rotated n. The lowercase e and lowercase a are interchangeable in many instances. Sometimes z and s work much the same. There are, of course, more.
Why would you want to recycle letters? Uniformity of appearance. It’s not essential, but it’s worth considering; the fewer the differences, the less the eye has to take in at a glance and the more quickly the message is received.