It's the difference between "I drive 65mph on the highway" (speed) and "I drive 10k miles a year" (usage). If you only drive 20mph everywhere, you won't likely put 20k miles on the car. If you don't drive much, even at highway speed, you also won't put many miles on the car.
If you use 1 gigabyte per month (seems very, very, very low - are you sure they didn't say terabyte? that's 1000 gigabytes) in theory your 1 gigabit/second connection can supply that in 8 seconds at peak. (By comparison, I don't stream much netflix, stream a lot of music, and play video games. My monthly usage is around 120 gigabytes/month, it goes from 100-150 depending on the month. It's not counting TV you watch, as far as I know.)
The main reason to check out the higher-speed plans is that they often come without data usage caps, or very high ones. But if you are really only using 1 gigabyte, you're going to be well inside any monthly data usage cap. My Xfinity plan's monthly cap is ~1200 gigabytes (= 1.2 terabytes), more than that I'll start hitting overage charges. So I'm using like 10-12% of the usage cap most months.
This is their 1.2 gigabit/second plan, mostly because that was what had the best promo rate when I renewed it last year. I used to have something like 300 megabit (= 0.3 gigabit / second) before that but the 300 megabit plan was either discontinued or actually more expensive than the 1.2 gigabit plan when I renewed, I forget which.