Majoru Oakheart
Adventurer
Don't worry, I can't grasp it either. There's basically no reason at all for you to use a katar in your offhand except for some strange desire to use one.For example I had a debate fairly recently with one of those players regarding my use of a Dex based offhand weapon on my strength based warrior. I had chosen the weapon for flavor sacrificing the damage because I wanted m character to have a certain feel, (using a long sword in his main hand and a Katar, which we had house ruled to use dagger stats as katars don't as yet exist in 5th in his offhand) the player in question simply couldn't grasp why I would chose a weapon my character wasn't built to use when other options were available.
There are certain times when role playing choices make sense to me. I've argued that this whole thread. There are other times when you are just gimping yourself on purpose. There are a bunch of weapons with similar flavor to the Katar that would do a lot more damage.
It's times like this that I always ask people "So, imagine you go right before the enemy. You attack and you bring them down to 2 hp. They go and kill one of your allies...or you. Was it worth it to make that roleplaying choice? Do you think your character would have chosen a weapon they were bad at over one they were good at knowing that there were life or death stakes?"