Back end of the NBA draft is basically the NFL version of a 7th rounder.
I think it charts more like this:
top-5 pick: first half of first round NFL equivalent
4-10: late first round NFL equivalent
11-14: 2nd round NFL equivalent
High non-lottery pick (15-20): 3rd round NFL equivalent
Late non-lottery pick (20-30): 4th-5th round NFL equivalent
Second round pick: 6th-7th round NFL equivalent
Which is another way of saying that any pick within the top 20 of the NBA draft has a lot of value. In the teens you have a great shot at finding a solid rotational contributor--anything from the 4th to 10th guy in your rotation. Given how rosters are built in the NFL with most contenders having 3 guys taking up a ton of cap space, it becomes basically a necessity to have solid role players playing on cheap contracts, and hitting on late first round picks and any second round picks is a great way to do that.
Plus, with top-20 picks there's a very real possibility of knocking it out of the park and getting a franchise player: Myles Turner (11th in 2015), Devin Booker (13th in 2015), Giannis (15th in 2013), Klay Thompson (11th in 2011), and Kawhi Leonard (15th in 2011) are all either franchise guys or the next tier down.
Once you look at picks 20-30 it gets a little murkier, but there is still a ton of value there if you pick well. I'd liken it to rounds 4-5: the good teams find genuine plus talent there, the solid teams find contributors, and for the ****ty teams it's basically a throwaway because they suck at scouting.