Why Everything Data+ and $10 Premium Data on the same plan

I've tried to research what Everything Data+ offers and compare with the $10 Premium Data for each line and I'm not finding a reason to have both. Correct me if I'm wrong but Everything Data+ for our family gives us 100GB data, then slowing to whatever. We barely use over 100GB, but with four lines, it happens - and yet we get notices, usually when we cross the 50GB threshold saying that our service may slow. That doesn't jive with what's advertised about Everything Data+. Is this how I was drawn in to tacking on the $10 Premium Data per line? I can't even find what the Premium Data charge ensures for how long, or how much data is included. ¿Qué pasa?