The point is that a tier1 PC(L1-4?) & a tier2 PC (L5-9?)/tier3 PC(L10-16?)/tier4(L17+) face different challenges. There used to be
a mechanic that the GM could use to adjust the values as appropriate (which is pretty much what the dc5-dc43 table shows) but 5e moved away from that kind of thing in favor of
bounded accuracy & what
@Lanefan eloquently summarized as overusing advantage/disadvantage
"to the point of absurdity" resulting in a situation where players feel cheated opts to give bob DCs that enable
competence porn rather than everything just having the world turn into an endless chain if
Absurdly Ineffective Barricades once the party crosses the bounds of "bounded" accuracy.