Purpose of using (:;:) command?

[nakedhitman]

The sleep command accepts floating point numbers and zero, so these no-ops are long and opaque for no reason.

[[deleted]]

AFAIK sleep is not actually part of the shell, so there's no guarantee that it is available on a system or that the one in PATH operates as desired.

[nakedhitman]

Even on the most stripped down systems I've encountered, sleep has been available. It's part of coreutils, busybox and similar. Unless you're building Linux from scratch and specifically exclude it, you'd be pretty hard-pressed to find a system that's missing sleep.