GMAT Problem Solving (PS) | Re: What is the sum of all possible 3-digit numbers that can be

What is the sum of all possible three digit numbers that can be constructed using the digits 3,4,5 if each digit can only be used once in a number?

The explanation:

6 ways to arrange the digits: 345, 354, 453, 435, 543, 534.
Each digit appears twice in the hundreds column, twice in the tens column, and twice in the ones column.
Because each digit appears twice in the hundreds column, you have 3+3+4+4+5+5 = 24 in the hundreds column. ***this is where I lose it. why are we adding the the digits in the first place and why twice?