Having both worked in startup/tech companies and ad agencies, one of the most valuable skills I've learned is being able to decipher and debug someone else's spaghetti code. Clients shift agencies all the time and sometimes you're just given the entire codebase for site, with no documentation and no help offered from the previous agency, and you have to make an update to it or learn how some vital part of it works so you can recreate it in the new site. That's happened to me quite a few times and it made me a better debugger and coder.