Sure it was not legally needed, but President Obama is not taking any chances. Tuesday, Chief Justice John Roberts mistakingly rearranged the word "faithfully" in the Oath of Office lines, which triggered an unexpected pause from Obama, then a repeating of the lines correctly from Roberts, but then a misspeak from Obama. Anyway, on Wednesday night the two did a reswearing-in.
Though Obama was actually made president at noon on Jan. 20, before the Oath anyway, there are some who might challenge him at a future date, or threaten him, and use the flubbed swearing-in as an excuse to do some harm. Good on Obama for acting quickly, and getting this out of the way. With a few clusters of nutcases looking for vulnerabilities, it's best to minimize the risk of threats like that.