Why shouldn't a GET request change data on the server?

Question

All over the internet, I see the following advice:

A GET should never change data on the server- use a POST request for that

What is the basis for this idea?

If I make a php service which inserts data in the database, and pass it parameters in the GET query string, why is that wrong? (I am using prepared statements, to take care of SQL Injection). Is a POST request in some way more secure?

Or is there some historic reason for this? If so how valid is this advice today?