jmartkdr2
Hero
I'd build on this by saying: those are the assumed midpoints of character power - they did try to design a game that worked over a fairly large range, to include multiple playstyles. So while it's true the game is balanced around a +3 ability mod (and few but not zero magic items) to start, it's balanced for a range of +1 to +5 (and zero to piles of magic items).Any D&D edition with 'encounter building' rules must necessarily have some concept of how powerful PCs are. The 5e encounter design metrics seems to assume standard array or point buy, so that characters begin with a +3* (or occasionally +2) in their primary attribute, with very minimal magic items (but able to harm the rare creatures hit only by magic weapons), no multiclassing & no feats.
Since most games have a fair number of magic items, feats, and often multiclassing, the Challenge ratings & encounter building systems tend to be fairly accurate at low level, but create very weak challenges at high level.
*By either standard array or PB, a PC can start with a 15 (+2) in primary attribute, which can very often be increased to a 16 or 17 by racial bonus.
How well they achieved 'it works over the whole range' is something reasonable people can disagree on, and partially hinges on whether you think adjusting your encounter building to the party you have is 'hard' or just 'part of being a dm.' (Not to mention how well you think this is presented in the books.)