Because an indefinite integral is an antiderivative, and you want to find all functions whose derivative is the given function. If you add a constant to a function it doesn't change the derivative, so the antiderivative needs that constant in order to be general.
For example, an antiderivative of 2x is x^2, because the derivative of x^2 is 2x; but the derivative of x^2 + 17 is also 2x, and in general the same is true for x^2 + C, for any constant C. So for any C, x^2 + C is an antiderivative of 2x.
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