That's indeed part of it. Another part is that working at another company usually gives you new perspectives and you learn more. So having been in a lot if companies and in turn having seen a lot of different approaches counts towards your experience, which also justifies a higher salary.

I actually got a 10k raise in one company, but only because I really didn't earn enough before. When I quit a year later anyway for another job (which offered me another 14k raise), my employer made a counter-offer of 30k. I did not take it, but at least it gave me a perspective how much I apparently was really worth to them.