The person using the class you created has no idea as to what is the implementation. He might use User.ID over and over again without knowing each of those is a DB call.
You see, 99% of the time, properties are nothing more than variables with an extra line of code at best, so developers treat them as such. It's considered good practice to refactor a property into a method if it's expensive in any way. You never know what a method is hiding, and (good) developers conserve on calling methods when they can cache results from previous callings.
