Paper currency is, of course, the ultimate protector of anonymity, for making ordinary payments at the retail level. It is, thus, a measure of how valued is privacy in our system that inroads into the use of currency have been slow, and halting, in the face of technologies one would assume would have quickly buried the presumed inefficiency of paper transactions.
To be sure, checks leave a paper trail which can compromise privacy, but it is a less efficient and accessible trail than when available newer technologies are used.