A FB is effective largely by it's velo and also (but generally less so) by it's location and movement. A CB is effective largely by it's movement and location and also (but less so) by it's velo.
A good velo FB will keep hitters honest and make decent off-speed offerings more effective. Without a good velo FB, other off-speed offerings are less effective. The hitter knows there is time to sit.
Movement and effectiveness of a CB can often be adjusted with minor tweaks to grip, release, arm angle, etc. Minor tweaks won't change an 84 mph FB into a 90+ mph FB.
A 90 FB during recruiting is likely to become 92-94 in college. An 84 FB is much less likely to become 92-94 in college.
And on and on... a good velo base is what sets up everything else.
This is not to say that a P with a particularly sharp CB can't be effective leaning on that pitch.