First, the source of the content of the work, as xpnine said, some versions of poetry are different.
The second is the source of characters, especially seal script. Because the development of characters is more complicated, seal script belongs to relatively old characters, and the number of words is also relatively small. Some modern scripts did not exist in ancient times. If you write a word that is used now but not used in ancient times as a seal script, it is more complicated to find out the source of this word and so on. There are also seal scripts, especially Oracle Bone Inscriptions, which have structural variability when writing. There may be many different ways to write the same word, and some words are easy to make mistakes. Without the knowledge of philology, a simple description may not be correct. Moreover, the interpretation of some Oracle characters is controversial in academic circles. The judges of the calligraphy competition have limited written knowledge. In order to avoid typos in his works, it is required to indicate the source of seal script. For example, indicate which dictionary and description the word comes from (such as Yin Ruins Book Deed).