Yeah, a Gerber file is supposed to be, in essence, a flat, monochrome, dimensioned picture -- but its structure belies the method in which it was created. Like PDF, SVG and other vector formats, you can infer things about the original source, beyond what the final shape would suggest.
Example: if you place an SMT component in the middle of a copper pour, not only do you get the fill of that pour (which itself might be constructed from lines or something), but a redundant pad is left in there, which lines up with the mask and (if applicable) paste openings. PCB houses have software that detect these objects. If you have something inconsistent (a pad object with no mask opening, or vice versa), they almost always flag it, just to check and be sure.
Tim