why 26mhz

Why mostly communication system adopt 13MHz or 26MHz for
reference frequency ?
What's the advantage or some other history ?
Thanks.

I instead see often on communication systems and test equipment a reference frequency of 10 Mhz.Why?

ITU (International Telecommunication Union) is allocating the clock frequencies. The main idea of clock frequencies allocations is that the fundamental or its harmonics to do not arrive in allocated communication bands.
GSM (Groupe Special Mobile) by common agreement decide to use 13MHz, and lately 26MHz that is double of 13MHz.