On Windows, the BackSpace key deletes a letter, and on the Mac, the delete key does the same.
On Windows, Ctr + BS deletes a word instead, and on the Mac, Option + Delete does it.
Why doesn't it delete a word by default? To me, setting deleting a letter as the default behavior doesn't seem to be a good design.
You would press the delete key when either (a) you misspelled a word or (b) you want to replace a word with another.
In the scenario (b), it's obvious that deleting a word is more convenient. If it deletes a single letter at once, you have to press the key multiple times.
In the scenario (a), you may want to delete a single letter if you exactly know that you misspelled at the last letter of the word. If the misspelled position is 3 letters away from the cursor, you have to press the key 4 times, and then type the rest of the word. At this point, you would probably want to press Option + Delete to delete the entire word and then type the word from scratch. Since you are used to type word-by-word, typing a word from the scratch is probably easier than continuing from a midpoint.
I almost always use Option + Delete when writing, and I wonder why neither Microsoft or Apple didn't this as the default behavior of the key so that you don't have to use the key combination.