Geometry matters for several reasons.
First, It means you can arrive at a shape with the minimum number of polygons... I don't need to explain the need for a low poly count right?
Second, lighting is calculated across the surfaces of the polygons. Because of things like rounding errors and aliased pixels, a proper geometry will render better than one with a lot of irregular faces.
Lastly, it is easier to texture a model when the UV map is easy to understand and figure out as opposed to being a jumble of anomolous polygons with no rhyme or reason.
Here, have a look at your render. Even though it is small I can point out problems that are being caused by the geometry issues:
Right off the bat, do you see the line running down the rear pistol grip that's not supposed to be there? That's caused by the geometry issues your model has.
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