It's to make what was done (which is important to the paper) seem important and make who did it (which is less important to the paper) seem unimportant.
The interesting thing to me is that in mathematics journals, the universal pronoun is "we." The reason (I've been told) is that "we" represents the collaboration of the author and the reader to understand the results and proofs in the paper. This makes sense to me, because reading and writing mathematics is a skill entirely apart from most other types of discourse. (Of course, when I say "mathematics," I mean to include fields like theoretical computer science and others in which discourse is of the "theorem, proof, discussion" form.)
That is interesting. I instinctively use "we" when commenting code or talking myself through performing a novel task, in both cases for the same reason.
