I don't know; it's probably the difference in weight of you carrying two water bottles vs. one. I wouldn't ever know the difference myself.

Edit: I misunderstood your earlier post. You're saying the 2019 model has disc brakes and the 2018 model has rim brakes? I would personally prefer the disc brakes. I don't need them either, and I resisted them for the longest time. But I bought two bikes this year with hydraulic disc brakes, after trying some of my dad's bikes with disc brakes this spring. Man, those disc brakes are smooth and powerful. All the rest of my bikes have rim brakes, but I hardly ride them anymore. To me, disc brakes don't meet a particular need...they just make the ride so much more enjoyable for me. Absolutely not a necessity, though.