Chess trainers can train with many positions

Many trainers' selection of illustrative material is highly informative, particularly in their methodical illustration of different types of middlegame tactics and endgame categories (though how often the trainers' illustrative positions are likely to come up in over-the-board play remains an open question).   We should appreciate their use of multiple move-by-move diagrams to illustrate especially pertinent or seminal positions, such as B-and-N checkmates, some basic K-and-P endgames, and particularly fascinating middlegame positions, such as those involving Queen sacrifices.  Considering the vast quantity of possible examples they could have chosen, many chess trainers have done fair justice to middlegames and endgames at the introductory to highest level.