(a) even from a solely economic standpoint, you're increasing the value of the artist's work and theoretically increasing the chance that somebody, deprived of the bargain, will hit it first hand,
(b) the money goes, in theory at least, to a worthy cause rather than nowhere,
(c) assuming that the donator is playing fair, which is a civikized society's assumption, they don't retain a backup copy of the contents of the CD and so the total number of copies in circulation and/or available for simultaneous play is unchanged, much like library books,
(d) charity shop music purchases are, because they're cheap, more likely to be impulsive/less specific and therefore introduce you to a corpus of work you might not otherwise have fully explored, creating the potential to make you a fan in future,
(e) also, you get to rip it with the encoding settings you want, not what some dweeb half way across the Internet thinks is best 😎