This lets you quickly traverse through the history of a file. Another good tool for digging into the history is the -S flag to "git log", which finds commits that added or removed a certain string (there's -G that searches for a regex, too).
All in all it has a negligible impact on the readability of the code. It's mostly aesthetic for me. It's ugly and only solves far fetched problems.
Do you scratch your name and SSN into the side of your car? What if your title blows away in the wind on the same day that city hall burns down destroying all ownership records?
Also, if you're using GitHub, their Blame view also a button that looks like a stack of rectangles between the commit information and the code. Clicking that will essentially do the same thing command-line git operation above.
(I'm embarrassed I didn't think of it before)
I've been in the industry less than half that time and it's happened to me. blame will tell you the last person/commit to touch that line of code. To find out when it was originally written, I may have to (in some cases) manually binary search the history of the file.