You have mentioned most of the reasons people do this. In some cases, they might also be coming from schools where the full four years of BS will allow them to complete a progression, in math perhaps, that is better for college applications.
Also, at a BS where this is common, the AO might suggest it to help a student adjust better. While a younger student might fit in fine at one school, it might make for an awkward social fit at another. AOs understand that a good “fit” can make a difference in whether a student thrives or not. If the bias is toward older students – just as it is in some zip codes for kindergarten-- it can help to reclass. Remember that many of the students who don’t reclass may already be old for the grade owing to kindergarten red-shirting or where their birthday fell in the kindergarten cycle.