Unless you are very fit and have a perfect body shape, a very well tailored shirt/turtleneck can look significantly more flattering than an off-the-rack item. It'll sit well when you're in a neutral pose and stretch or pull appropriately when you gesticulate.

You correctly interpreted the point I was making — Steve Jobs treated his casual look as seriously as others treat an expensive tailored suit. And the result means he's still signalling importance and success, without also signalling conformity and "old world" corporate vibes.