The game is likely balanced with the assumption that your main combat stat is 16
With point buy, you can get 14 or 15, but racial bonuses may well push that up to 16 (unless you're playing a combination that doesn't have complimentary skills, like a Strength based High Elf Paladin; not that there's anything wrong with that, that was my first ever character and I stand by it).
The various spellcasting examples seen in the PHB also assume that your spellcasting stat is 16 (which, for classes like Sorcerers and Wizards, is your primary damage-dealing stat, even if it's not used for a weapon). For example, for Wizards (PHB, pg. 114):
For example, if you're a 3rd level wizard, you have four 1st-level and two 2nd-level spell slots. With an Intelligence of 16, your list of prepared spells can include six spells of 1st or 2nd level, in any combination, chosen from your spellbook.
Paladins, by contrast, have a similar block of text (PHB pg. 84) that assumes a Charisma of 14; a Paladin's primary stat is often considered to be Strength, so in this case Charisma would likely be considered a secondary or tertiary stat, hence is lower.
The primary stat is often given in the "Quick Build" section that every class in the PHB has. For example, for Wizards (PHB, pg. 113):
You can make a wizard quickly by following these suggestions. First, Intelligence should be your highest ability score, following by Constitution or Dexterity.
And for Rogues (PHB, pg. 95):
You can make a rogue quickly by following these suggestions. First, Dexterity should be your highest ability score.
Incidentally, high Wisdom isn't a bad choice for rogues, since you can take the Perception skill and have a really good passive Perception for spotting all those traps and such... a good thing for a scout to have (which is the role rogues often end up in).
Also, the average of rolling 4 dice and subtracting the smallest value is 12.24, with the likelihood of rolling a 14-15 pretty good, leading to a 16 with racial bonuses: There's only about a 15% chance of rolling an ability below 8, with about a 56% chance of getting at least a 16, and rolling a 14 or higher 69% of the time (average 14.17 (σ=1.44)). The linked answer has a nice visual depiction of the 4d6d1 results. Based on the math behind it, the "average rolled array" for 4d6 drop 1 would be: 16, 14, 13, 12, 10, 9.
From this, I can conclude that the game is likely balanced around the assumption that you have at least 16 in your primary stat.