There are a number of varying reasons why having constantly good posture is important, even though you may think that this is only a cosmetic issue.
Firstly, when you stand upright, and stand with good posture, you will appear taller and thinner (biggest plus ever!), and obviously, with a lot more confidence. If you were always to look at photos of the same person and they were always slouching in the photos but standing tall randomly in one single one, they would probably look like two different people to you, and you would also have a different opinion of them both.
Bad backs can be seen from any direction you look at someone. From their posture (or need for posture braces and support) to their poor stride. These people would likely not be using a good office desk and one would assume they would need to work more on their ergonomical environments in every facet.
Secondly, when you are slouching, you are also not getting enough oxygen into your body (believe it or not), which means that lack of posture actually affects your energy levels as well as everything else. As a result, you may actually notice them gain weight and become more easily tired when they are not prone to standing or sitting upright.
In fact, they may also recommend that you have a posture support brace properly made for you, which could set you back hundreds or even thousands of dollars, which is something none of us want to deal with.