In all the years of play, and all the salty tears from peeps on both sides of the debate: I don’t know if I’ve ever heard a good reason for their existence.
Fitting a WCS on a hauler makes sense. Not really much else though.
Fitting them should reduce your Max Targets to ZERO.
Remove any Local Repair Bonus.
And negate the ability for the ship to apply ANY boosts.
It’s not about crying to CCP or catering to any specific style of play. It’s about sound game mechanics that work. The current state is better than it was, but it’s still not where it needs to be.