Dha is the voiced aspirated dental, breathy voiced, and it fills the fourth slot of the dental class. Unlike the corresponding letters in the palatal and retroflex classes it is common, standing in a large family of frequent roots and in several grammatical formations, so it is written constantly.
That is a useful corrective to the impression the table gives. The fourth slot of each of the five classes holds a letter of the same phonetic description, and the five of them differ enormously in how often they are needed: one is met on every page and another may not be met at all. The arrangement is by articulation, and the vocabulary distributes itself across the grid unevenly, which is a fact about the language rather than a flaw in the analysis.
In Buddhist texts specifically the letter is unavoidable, because it opens the word dharma and the long list of terms built on the same root. A page of doctrinal writing carries it many times over, and its shape is among the ones a reader of such material recognises fastest.
Like the other three voiced aspirates it has a character of its own rather than being derived from the plain voiced stop, which keeps the structure of the class legible on the page.