It tells us if there is possible crossability between two species. This is normally only possible with taxa that are of the same chromosome number. The most important reason is the potential economic or medicinal contribution of any plant. Our domesticated varieties are highly inbred and not overly heterogeneous in their genetic background. They become, in effect, human creations.
Corn is an excellent example.
Corn has a diploid chromosome number of 20. The inbreeding and selection have produced a crop that is highly dependent upon humans for the completion of its life history. The leafy tissue over the seeds is designed to protect them from predators. The seeds do not naturally disarticulate from the ear. They remain on so that the seeds can be harvested by us at any time.