Practical implications of noting "equally contributing authors"
I would see such a step as a clear indication of authors contributions for the record, so as to make sure this gets accepted as truthful also in the future during promotions, job applications, etc. Technically speaking, in a CV one can write stuff even dishonestly disregarding co-authors (e.g., arbitrarily indicating the percentage of own contribution). This  kind of public indication of the contribution tackles the problem once and forever. In some fields the alphabetical order is a standard (mathematicians I know have that as almost a strict rule) and then you need a tool to make your contribution visible. Especially if your name is at your disadvantage in this game. E.g., when you are called Zhang and co-author paper often with some Adams or so. Also, if alphabetic order is desired for whatever reason and the more senior co-author happens to precede the student who did the actual work, such an indication might be desirable.

Another point is, some journals require such an explicit indication. E.g., Nature does that, although the statement of individual contribution comes as a paragraph at the end of the paper.

Finally, some national schemes for evaluation of research output require such an indication made in a publication archive system. Having it included in the paper also indicates that authors try to avoid any future disputes regarding individual contributions.