TLDR: battlecruisers were concepted to chase and sink heavy cruisers and they did that well battlecruisers in line of battle were never as effective as expected though without critical flaws they didn't do too terribly (HMS lion)the WNT made them more important due to the increased prevalence of cruisers and later on carriers which would need them for escortin RTW battlecruisers are good for the exact same purpose as real life which is sinking cruisers and providing a nice monthly supply of VP due to cruiser sinkingsBut just like real life the enemy also builds battlecruisers and if your battlecruisers are smaller than theirs yours is likely to loose and get sunk and thus the enemy gets to protect their cruisers from you and sink yours without you being able to retaliate before carriers unless you build your own bigger and better battlecruisersBattlecruisers made sense for the time period that they were constructed which is around 1907 to 1935 and found new uses post 1942 as carrier escorts though some fast BBs could accomplish this
The whole idea and reason for them is the fact that they are heavy cruisers but with battleship grade armament (an extreme example would be HMS glorious/furious) more normal examples would be a Kongo (1912) or ships like it such as HMS lion the larger the battlecruisers got the more armored they became
the battlecruiser concept also changed for the worse and they were built to be more capable in line of battle (something they in reality were probably not the most suitable for)
These ships essentially made light and heavy cruisers irrelevant in skirmishes/raiding
An example of this was the battle of the falklands islands in December 8th 1914 where two british battlecruisers three armored cruisers (very outdated and practically useless and also too slow) and a few light cruisers hunted down and sank two german armored cruisers and a light cruisers (with two light cruisers escaping)
The main reason the british battlecruisers exploded was due to Beatty who did everything possible to improve his battlecruisers rate of fire this included disabling some flash protections and storing powder and projectiles in the turrets beyond safe limits
What this did was just make turret hits to British battlecruisers blow them up coincidentally HMS lion who had a new crew at the time of jutland did not follow these procedures due to being rather new this also meant that when her Q turret got destroyed she dident turn into a giant fireball like HMS Queen Mary did
Battlecruisers post ww1 were still very relevant due to the WNT
the limit to new battleships or heavy capital ships in general meant that the number of cruisers was steadily growing in each navy of both the light and heavy variety
on top of this the cruisers became rather fast and rather powerful and the only way to reasonably sink one was with a battlecruiser (no cruisers would ever stick around and fight battleships and a cruiser against cruiser fight had the risk of either side loosing their cruiser and thus carried a very high risk)
For example in the 1930s the only ships in existence in the world capable of catching a 30 knot heavy cruiser was HMS hood, HMS renown or HMS repulse
Generally speaking this will also be shown in RTW
you can play the game without having a reliable way to kill enemy heavy cruisers but you just wont be able to benefit from lower enemy cruiser numbers in fleet battles or monthly victory points form sinking cruisers
the enemy might also use them as raiders and starve you with heavy raiding cruisers and without battlecruisers you have no counter for this