The idea of tweaking the chances is to allow you to have higher control of the situation.
For example, you can lower the trigger chance of your defensive cards and then you can target the enemies and hope that your higher chance attack cards will trigger on them and the heals will be left for the final turns and you can cast them on yourself with 100% certainty.
Or, if you lower your character's target chance, you can play your attack cards first in hopes that they'll target the higher chance enemies.
I was working on an update to make it more visible through color-coding the chances, but couldn't upload it in time. In the uploaded version it is slightly annoying to have to read all the chances to come up with the optimal plan.
("haphazard" - first time I'm seeing that word, but I think it describes my game jam dev very well :D )