You are not an obtuse person. I know this.
Since you need an example---let's use this:
If the play breaks down or coverage busts - Justin can take off and use his legs. When there are 7 defenders in the box and both the A and B gaps are overloaded - where is the RB supposed to go??? Plays like that result in no gains, losses and, ask Mendy, fumbles.
Let's be 100% absolute crystal clear...
Najee IS NOT Barry Sanders or even Jalen Warren when it comes to burst and elusiveness. Heck, Patterson is more elusive than Najee. But...
When it gets cold and the field is in bad condition - I bet you'll look for your mudder over that elusive back.
If we somehow can keep both Najee and Jalen - we have a good problem to have...