Anyone who says that doesn't know what they are talking about.
Imagine for a moment you needed to move 50 liters of water from one place to another. If you just use your hands, its going to take a lot longer carrying it piece by piece, slowing you down as you make multiple trips. You could probably load it all up in a car and be able to take it all at once, even have a bit of space left over. Or you could take a tractor trailer to move it.
RAM is basically the same way. Your system, unless you are doing unusually heavy amounts of work that only about 5% of the computer using population actually do, will only need 8GB of RAM. If you have 16GB, it goes unused and just basically sits open doing nothing, but wasting power. If you had 2GB of RAM its a bit more like the 1st scenario in my example and you really could use more, but with 8GB you are at the perfect amount.