Why are 9mm bullets rounded while .50 caliber sniper rounds are pointed?
Although correct, there’s more to this.
Some bullets are shaped not just for their passage through the air. For example some bullets expand (hollow point bullets,as one example).
The Geneva Convention signatories pledged not to use expanding bullets. This is not a concern for police forces, as the Geneva Convention doesn’t apply inside any country. The militaries wanted the same effectiveness as expanding bullets without violating the Geneva Convention. So, the .45 ACP is designed to tumble inside a body, called a dum dum bullet, while the 5.56 NATO round is designed to fragment inside a body.
Alternatively, supersonic rounds, like supersonic airplanes need a different design than subsonic rounds.
Finally last specific example for this post, there may be a desire to limit the range of a bullet. This actually occurred between the M1 and M2 round in the US M1 rifle ammunition used in WWII. Rather than extend the safe zone around the rifle ranges used, as the new rifle had a significantly longer range than the rifle it was replacing, the Army redesigned the bullet so it wouldn’t travel farther than a round from the previous rifle. The M2 ammunition was used for most of the combat in the war. Similarly, a person selecting a deer rifle cartridge or self-defense round may want a bullet to slow down and drop to the ground relatively quickly, after it has passed the desired shooting range.
There are a number of other issues, but they are more special cases.
Pointed bullets are more aerodynamic while rounded bullets gives more weight for the same length of cartridge. So for higher speeds, longer ranges with flatter trajectories you want a more aerodynamic pointed bullet while if you want a small light cartridge that fits nicely to a pistol that is used to fire slow heavy bullets at short ranges you want the extra weight and short length that a rounded bullet gives you.