It provides a way to give new beats that aren't normally available to regular instruments like a trumpet or a flute. They can turn sixteenth notes into 64th notes and even more than that. They can also provide a way to express oneself through a place of repeating over and over again that would be extremely difficult on a person playing themselves.
A musical instrument is something that is used to create music. Synthesizers do exactly that. Songs can be performed with them just like a guitar or the human voice. It may not be a conventional instrument that the majority of people are familiar with, which is probably why this question arose, but there is no denying that the synthesizer is a real, viable instrument.
A musical instrument is "a device created or adapted to make musical sounds". A synthesizer fits the mold.
The synth is used in many musical styles, and can even spice up a remix of Beethoven. Its sound is not alien if you know what you are doing. They are also very adaptable. As musical tastes change, so do synthesizer presets. A C chord on a piano sounded exactly the same then as it does now. Blah. A keyboard can create new sounds that people once only dreamed of.