KaeligRoperhe
The Apocalypse-class Battleship is a very old design, its main weapons are broadside batteries of lance weaponry. They were extremely powerful (and are extremely powerful in the 40-41st millenia, even more powerful than the re-invented ones used on the Victory class, more on that later) but used too much energy. It leads to draining power from the engines while using the lances on full power aka on the highest distances available. That's why they are often (almost always) being used on medium distances, because if the target isn't crippled via long-distance sniping, ship becomes extremely vulnerable, due to the fatal lack of maneuverability. Additional lances weren't an option for obvious reasons, but ship needed additional weaponry for medum-close distance combat. That's why it has macro-turrets. The technology of these lances was lost after the heresy and Apocalypses are now quite a rare sight. But Mechanicus wanted to make more "battleship-snipers" that's why they kinda re-invented big broadside lance batteries. They were installed on the hull of one of the most common battleships of post-heresy Imperium - on the hull of Retribution-class battleship, which already had lance turrets on top. New lance broadsides weren't nearly as powerful as the ones of Apocalypse, but were able to fire at maximum distances without overloading the ships reactors. This ship design, which uses ONLY lances is relatively new and is called a Victory-class. If you still don't trust me, check Warhammer wiki, and data of Battlefleet Gothic, where you can find more detailed information on this topic.