It is necessary to do these "trivial" endgames in order to do the practical ones. If we agree KPPP v KB is a practical endgame (seems so), then we need to know the outcome of all KQQQ v KB endgames, since it might be the case that the only winning line is to reach a winning KQQQ v KB endgame. If you want to evaluate the "practical" endgames, you need to know all the final positions that could lead to them by retractions. Notably, pieces can unpromote into pawns. Pieces can uncapture other pieces, leaving an extra piece on the board.
Now what is a retraction? Pieces move backwards, so everything moves as in normal chess except pawns. Do this for all positions, and we know the ideal outcome of all of them. In doing so, we know a possible outcome of the resulting position (by playing forward to the final position again). Then from each, we "retract" (take back) moves that could have been played. First, every possible final position is exhaustively listed.
Tablebases are generated by working backwards from the final mating/drawing positions, "retracting" moves rather than playing them forward. We have to understand how tablebases are generated to see why. There is a contradiction in terms in "only" generating practical endgame tablebases.