Players tape baseball bats for a variety of reasons, all of which are designed to increase performance or extend the usage time of the bat. Taping the handle or near the bat's sweet spot can each contribute to your game as a player, but for different reasons. Some players tape metal and wood bats, but it is far more common to see a player tape a metal bat.
Get a Grip
Players put tape on the handle of wood, metal or composite bats to improve the grip they have on the bat. Wood bats are made with no grip, so most players either place a sticky substance such as pine tar or tack on the bat, or use tape to make the bat easier to hold. Metal bats come with a grip attached, but some players prefer to use their own tape for a more comfortable or better grip. Composite bats can vary in design, but adding tape if your bat doesn't have a grip allows you better control.
Sweeten Up the Spot
Wrapping tape around the sweet spot of a wood baseball bat can increase the life of the bat. The sweet spot is located around the midpoint of the barrel, where the bat is the fattest. Wrapping tape around this spot can help make the bat last longer, and if there are already little cracks or peeling, the tape can keep the bat intact to use during practice or drills. You won't be able to use a bat with tape in this location during a game, but taped bats are suitable for practice.
A narrower taper or grip decreases the weight of the bat and allows you to rotate your wrists faster and generate more bat speed. The grip that metal bats come with is usually much thicker than a layer of tape, so some players elect to remove the manufacturer's grip and apply tape for this reason. The tape usually keeps the handle very thin and much easier to maneuver when holding and swinging the bat.
Bye Bye to Sting
Some players apply tape to a metal bat or apply the tape over the manufacturer's grip to make the taper or handle thicker. More tape can reduce sting and absorb more of the shock when the bat makes contact with the ball. Adding tape and thickness also makes the bat slightly heavier, which is another goal that some players have when adding tape.