While watching an anime series with a friend, he pointed out that "all anime play their theme song during the final fight." Now, I doubt it's "all", but sure enough, the main theme song of Digimon Tamers and the (first) OP of Fullmetal Alchemist: Brotherhood play during the final epic clash in their respective series. (And I'm sure there are many other examples.)
Do most anime do this? Is there a reason for it (as opposed to just playing a new epic piece)?
Tvtropes call that a Last Episode Theme Reprise, and mentions that it is used in video games as well:
Since it's standard practice to make a Theme Song as awesome as possible, said theme likes to find its way into a privileged position in the final battle. At The Climax of the series, when things are at their most epic, the theme gets played over the show itself. Typically we'll get to hear the verses not played in the opening, and sometimes the whole song is strung together for once.
The opening or closing theme can be used. Often this serves as an ultimate Theme Music Power-Up. Often it serves in this capacity for the show itself. The creators seem to be claiming their own Crowning Moment of Awesome with this music.
This is also common in Video Games, the most common being a reprisal or orchestral version of the main theme over the final dungeon or ending sequences.
I would say that it is at least not uncommon, since the page has an extensive list of titles that use this trope, to name a few:
and many others. As for Is there a reason for it (as opposed to just playing a new epic piece), usually there isn't. If it is significant, however, it rather seems to be the Title Theme Drop trope;
