The letter and number drills are to fill in size requirements between common fractional sizes. Commonly the drill size used for tapping is intended to give a thread that only has 75% of its theoretical height; the hole is intentionally oversize by a small amount. This causes a negligible weakening of the thread but trying to tap a full height thread by using a smaller drill dramatically increases the load on the tap and in steel may be impossible with small taps. With some thread sizes using a fractional drill will be too loose or too tight.
Actually when starting out it is sometimes a good idea to use a slightly larger drill and some tappin charts give an alternate for a 65% thread. This makes it much easier for a novice to learn the feel of tapping without breaking too many taps. It is simple to compensate for the reduced strength in the tapped hole by using a slightly deeper hole and a longer bolt engagement.
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