Comment by dgacmu

I mean, yes, if you're designing a microcontroller you need a clock source, but from the perspective of implementing a circuit as a more normal designer, the UC has it already. And lots of small UCs have an internal clock source so you can omit the external xo unless you need a particular frequency, accuracy, or in some cases a precisely timed micropower sleep. For example, the sub-$0.10 32 bit PY32F002A can do 24MHz internally -- but I would bet it doesn't have great temperature compensation or other things you'd want for timekeeping or being a good frequency ref. (But you're not going to get that out of $0.20 of components anyway.)