Tennis bags are used by all professional players to store anything they may need during a match. All players have some sort of bag as they need to have at least one spare racquette in case their first one breaks. Players can use bags to store spare tennis racquets, sports drinks, shirt replacements, grip replacements, shoe replacements, and sock replacements.
Over the course of a long match, players' strings pop, racquets are smashed in anger and tennis grip falls off. This is why players always have backup racquets. Along with racquets, players carry spare clothes that they can change into when they get very sweaty and begin to feel uncomfortable. Energy drinks as well as tennis grips are also stored in these bags in order for players to be equipped with all the proper things they need to effectively compete in a match.
Most players have sponsorship deals. This means that they are paid by their sponsors to sport specific bags to enhance the brand's image. For example, Rafael Nadal will always be seen using a Babolat bag because he is sponsored by them. It's a win-win for the players because they get a nice bag for free and are even paid to use it!