Dustin Curtis on why and how designers manually tweak icons for specific pixel sizes:
In the Markdown Mark repository on Github, I had initially included only 12 static PNG images of the mark at different specific sizes. Several people asked me why I had done this instead of including a vector graphic, and the reason is actually extremely important. This is something everyone should know, especially people who are not designers.
I found his example with the Hulu logo quite striking: