Or you trade down.
But traft boards and stictly numerical.
It's more like
R1, Tier 1 (4-5 players)
R1, Tier 2 (maybe 5-6 players)
R1, Tier 3 (maybe 5-6 players)
etc
You always have choices within Tiers, but it the gaps bewtween tiers where teams sometimes reach. Say Your Rd1/Tiers 1-3 players are all ok to take at pick 18 but when you get there only one is leaft and it's not a postion of need. You really need to trade down to get better value there. If a Tier 1 player is still on the board, you may end up taking them regardless of need. No trade down..nothing, just take them because the value (according to your board) is overwhelming.
Otherwise, the trick is use BPA but try to match value with need.
If BPA is QB at 18
It may be more valuable to trade that 18th pick and get two or three BPA picks later that also match need.
Same value but better use.