Paper Reflects Light
Paper reflects light. Light travels through the layers of transparent process ink, bounces off the substrate, and travels to the reader’s eyes. The color or tone of the paper influences the reader’s perception of the ink colors. That is, a yellow-white (or cream, or natural) sheet will add a yellowish tone to the inks, which may be incompatible with the subject matter (such as people’s faces).
Interestingly enough, if you print 4-color process inks on a transparent acetate sheet (instead of paper), there is nothing behind the process ink to reflect light back to the viewer. So the colors will be very dull, if not difficult to see at all. To remedy this problem, printers lay down a background of white ink behind all 4-color process inks printed on clear plastic using an inkjet printer. This makes the process inks stand out.
So don’t underestimate the powerful influence of the substrate (paper or clear film) on the printed images.
Paper Density, Bulk, Caliper
Paper is composed of wood fibers, chemicals, and lots of air, among other things. (Paper is similar to a sponge. You can squeeze out the air or let it puff up.) A high-bulk paper and a more dense paper may both weigh the same (measured at, for instance, 500 sheets of 25” x 35” stock), but the high-bulk paper will feel thicker. (You can measure the thickness or caliper with a micrometer.) There are several benefits to using a high-bulk paper, which has not been flattened as much as a lower-bulk paper by the polished metal calender rollers on the paper-making machine.
These calender rollers flatten the paper and give it a harder surface, which allows ink to sit up on the surface of the sheet and makes for crisper photos (you can use more ink and a finer halftone screen than with an absorbent paper).
You can also give the reader more of a perception of quality with a higher bulk sheet (thicker paper feels more substantial). Or you could save money shipping a large job (let’s say 60,000 perfect bound print books) by using a lighter press sheet (perhaps 60# rather than 70#) that is of a higher bulk.