A good offensive that encircles and destroys 12 enemy divisions isn't just a nice maneuver, but it represents the destruction of months of production and training. But that's also when Hearts of Iron 4 is at its most enjoyable. That's much harder to do when the war is at its height, enemy bombers are striking your industries, and the consumption of equipment begins to drastically outstrip production. Do it poorly, and units will slowly wear-out as equipment replacements dry-up, while ceding one advantage after another to the enemy. Do your job well, and your troops will have a few key advantages when they enter combat, and weaknesses that you've anticipated and can compensate for. The joy of Hearts of Iron is making those high-level compromises between the army you want, the army you have, and the army you can produce. It's a rewarding problem to try and solve, but it's also a perfect one for the armchair strategist.
You want to leave production lines untouched for as long as possible, but that also makes it hard to replace obsolete weapons with newer models. You can always re-task factories to building new things, but Hearts of Iron punishes players who don't anticipate their needs effectively by making factories get more efficient the longer they produce the same thing. When you do get the designs you want, you need to dedicate precious factories to building them, and I promise that they're probably all building something else that's war-critical.