I actually keep some crusty old code on github intentionally. Since github sorts your profile page based on last commit, it shows up near the bottom of the list anyways. By having gross old code, I can show how much I have improved and advanced myself.
Looking at code that I wrote a few years ago (like when I was first learning OO and Python) and comparing it to more recent projects is like night and day. It helps me to not feel so inferior on those days when I read too much HN and get too hard on myself -- and I would think that an employer would be interested in seeing growth and improvement.
Following conventions/best-practices. Not duplicating code unnecessarily. Using features of the language to my advantage instead of forcing the language to fit my program structure.
And also just an overall improvement in the quality of the code - less room for bugs, less 'hacky fixes', easier to maintain/extend/modify
Looking at code that I wrote a few years ago (like when I was first learning OO and Python) and comparing it to more recent projects is like night and day. It helps me to not feel so inferior on those days when I read too much HN and get too hard on myself -- and I would think that an employer would be interested in seeing growth and improvement.