I use Fed Ex a lot at work. I’d say about 50% of the time, if you send it “2nd Day” to a major city/hub it gets there the next day anyway.
What you’re essentially doing is giving them up to 2 days to deliver. If they get a heavy load of packages this helps them decide which to load first. If it’s a light shipping day, they’ll put everything on the plane/truck/whatever.
So, Fed Ex loads stuff in the following order:
First Thing (gets there before anything else
Next Business Morning (a.m. delivery)
Overnight (afternoon delivery)
If they can move your package up in priority they will (they have no desire to store things, after all). For some reason, when I send stuff “regular overnight” to Atlanta from Chicago it usually gets there around 9-10 am, but to DC it gets there around 4 pm. Maybe DC gets more stuff? And nowadays there’s probably more security on it.
I’ve also had 3-day packages arrive overnight instead of 3 days, too. And if there’s a blizzard or other weirdness, your overnight box might not arrive for 2 days. Like with the recent storm out east.