Why are there so many different measurements for drill bits?

First off, left-handed drill bits are ideal for removing broken fasteners. Nothing else is as effective. Ostensibly you would use the drill bit for making a hole into which you would then insert an easy-out, and then wrench out the fastener. In practice however the heat of drilling combined with the friction of the bit itself is often enough to back out the fastener.

The gauge and lettered sizes are anachronisms, holdovers from earlier practice. Owning an array of these bits isn't important unless you're in clockmaking or auto restoration, where you may encounter parts that have such odd hole sizes. In modern practice they don't have much relevance and can easily be overlooked as what they are, relics of a more colorful time.

Roger is correct.

Another example is drilling holes for 1/8" pop rivets.

A 1/8" drill bit can be a bit tight so ideally you use a numbert 30

screws and bolts have many sizes == and == often you have to drill wood to make pilot holes for screws == and == it is sometimes necessary to drill into concrete or brick and that requires a masonry bit and they come in varied sizes === it is not like you are hanging a large or a tiny item on the wall and you need the correct bit for the job ... auto work consists of varied sizes of drill bits .... SO, the true bottom line is much like writing a paper for class and you do not use a magic marker for that ,, you use the correct pen ... and that goes for drill bits .... the right tool for the job .....

here is an example

if you wanted to make a pilot hole for a 1/4 -20 tap you would need a .201 drill there is no good fractional equivalent so you use a number 7 drill

http://www.physics.ncsu.edu/pearl/Tap_Drill_Chart....

Because for different uses you need different sizes.