Whether it is useful or not depends on the specific situation of the paper. Generally speaking, no matter what the paper is, the third author can play some role more or less, and sometimes the role is just not obvious, so the third author is useful. Even if it is useless, the author has been promoted in the writing of the paper.

The significance and function of the third author is certainly not as good as that of the first author and the second author. If the paper is published in general, then the third author is really of little use, because it is not difficult to publish a paper in general, and it is normal for the author to complete it independently. In the evaluation of professional titles, the third author who published papers is almost not recognized.

However, if it is some high-level papers, such as core journal papers or sci papers, the significance and role of the third author will be even greater, because these papers are difficult to complete by themselves, and they have to go through a series of scientific research experiments, sometimes by teams. The first author is often a tutor or boss, and it is sometimes difficult for ordinary authors to become the first author. At this time, the third author is also possible. If it is a title evaluation, it can also get certain points.

So whether the third author is useful or not depends on the situation. If the author publishes papers in domestic general journals, he might as well try to finish them himself, which will play a certain role in popularization.