What's the purpose of $δx$ here?

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Why is the $δx$ here necessary? I'm going through Knuth's Concrete Math, and in the section on finite calculus, it says that it's akin to the $\mathrm{d}x$ of continuous integrals, but wouldn't it just be equal to one, since we increment by one when we're summing? I'm referring to sums of the form $\sum f(x)\delta x$