clock frequency

Hello, I was looking at a sensor example code (https://www.parallax.com/downloads/3-axis-compass-module-propeller-code) and I noticed that the clock setting is as follow

_clkmode = xtal1 + pll16x
_clkfreq = 80_000_000

Q1. I thought _clkfreq is for the external clock frequency, for the most of props, it's 5M Hz.. ? But, how is it possible to use 80MHz? If I lower it to 5MHz, the sampling rate would be decreased as well?

Q2. Why is external clock is used?

Q3. Also, it says

Serial Clock - I²C Master/Slave Clock (Clock 160Hz Default)

What is 160Hz about? Is it just sensor specification or is it somehow related to 80MHz and affecting sampling rate?

I'd appreciate if someone can help me out.

Thank you.