Sell me on light pen primary weapons

I can’t help myself from running the Eruptor, it’s been my primary for a long time. I like playing as a ranged support often, taking the SPEAR and keeping distance.

What’s the point of the light pen weapons? Why choose one over a medium pen? What am I not getting?

I want to like them, but I always go back to ‘wish I had the Eruptor fo this’ in my mind.

Help me understand.