In Heroes of Might and Magic games, you have few armies. Often one hero is transporting basically all the troops you have, while other heroes are running around without troops to gather stuff. The reason why you want to concentrate all your troops in one spot is because the more powerful you are, the fewer losses you will incur when fighting. Ideally you have a doomstack that kills all enemies before they can even kill a single unit of yours. Splitting your army in half and doing pitched battles with two heroes would be much worse, as you would lose too many units, and be weaker afterwards.

The problem of that is that playing this way isn't all that much fun. Pitched battles might be more fun, but you do your very best to avoid them. There is fun management gameplay involved, but actually not much tactics. Units in the Heroes of Might and Magic series don't even have zones of control, which further limits tactical options. Other than the clever use of the waiting mechanic to move twice in a row (something the AI in HOMM: Olden Era is good at), combat is mostly trivial if you do the management part right.

Having said this, HOMM: Olden Era is certainly a good representative of the HOMM series, building on the strengths of part III. There is a heavily scripted campaign, and a great single game mode, where you can choose the general layout of the cities without having to get into details of map editing. The game is fun for some time and true to its origins, but doesn't overcome the basic limitations of those origins either.