Arachin 4 -- All: Kohanim, Levites, Israelim, Who Else?
When a ruling is formulated as "All are obligated in a certain commandment, Kohahim, Levites, and Israelim (regular Jews)," the word "all" is not superfluous, but in each case it adds something.
ALL are obligated to hear the Megillah - adds those who are occupied with the Temple service; even they should leave the service and listen to the Megillah.
ALL can declare a symbolic personal value donation, according to one’s age and gender, as prescribed by the Torah , Kohanim, Levites, and Israelim. But that is obvious! If not these, then who? Rabbah says that this is a hidden answer to Ben Buchri, who explained that Kohanim never pay a shekel-donation (even though the Sages disagree). One might therefore think the same about personal valuation, and this had to be negated. Abaye argues that there is no connection, rather, it is an answer to a possible idea that since the Kohanim do not redeem their firstborn, perhaps they can’t do symbolic valuations either. Rava denies that connection also and says that ALL excludes the possibility that perhaps one Kohen cannot assign the discounted valuation that another, poor Kohen, has to give - and the answer is that he can!
Art: Giorgione (Giorgio da Castelfranco Veneto) - The Three Ages