Safety. If you want to keep your factory seats, you can get a Schroth QuickFit Pro harness and use a HANS device. I did this at my second event, as a novice.
I think E9X seats are terrible for track use (I have a friend who disagrees though so YMMV). FIA certified fixed-back seats are awesome IMO because they are safer and you can feel the car much, much better. I eventually ran my QFP harnesses with fixed back seats. It’s not recommended, but I personally felt safer. I have never had an instructor complain, and I always make sure they’re okay with my setup before they get in.
You can also run a harness bar if you don’t want a roll bar. A lot of designs are shit, some are good. The QFP can be used with a harness/roll bar. Schroth actually recommends one if you want to use a QFP with fixed back seats. I eventually did this because a used one popped up for sale, and I had planned to switch to six point harnesses in the future. I run six points now, and at this point, that’s as far as I’m willing to go when it comes to safety (harness bar, fixed back seats, six points, HANS, tiny fire extinguisher). The next step for me would be a full cage but I’m not ready for that.
Brey Krause makes good products for the E9X. They have a harness bar that can easily be moved out of the way for tires (there’s a thread in this sub forum, somewhere), seat mounts that have provisions for racing harnesses + factory belts, and sub strap mounts if you want to run five/six points. I think they make quality products. The seat mounts are very easy to use.
Make sure the equipment you have is the same as the passenger side. Don’t be an ass and get the whole shebang for yourself, while keeping the passenger side stock.
https://www.bkauto.com/mobile/Category.aspx?id=308