Paper sizing provides paper internal strength, making it more durable so that we can handle it without damage. There is a trade off made about resistance and paper characteristics. Most art papers have an internal size introduced during manufacture.
Chemical processes useful for photography are liquid so will wet the paper and sink into it. This may weaken it to the point of damage. The paper may be so absorbant that the ’emulsion’ sinks into the paper beyond useful range. This would be the case in a pigment process where the unexposed portion of the image must be washed away. If the unexposed portion of the ’emulsion’ won’t wash off, then there won’t be light tones in the image.
Most people automatically size papers for use in the pigment-colloid processes (gum, carbon). It is easy to test the paper for sizing need. Equally easy to check the suitable sizing procedure.

mix the base color with colloid. drop onto the paper. do not sensitize the mixture. allow it to dry, then wash it off. If this washes out, your paper doesn’t need any sizing. it is unlikely this will be the case. the ‘paint’ will likely stick to the surface fibers of the paper. Next step is to mix your sizing and apply. Run the ‘paint’ step again. If the paper clears, you have a good size procedure. With this known, you can proceed to make your carbons, gums, confident that any clearing problem isn’t because of paper sizing.