One reason for using the full flap setting is that it gives you a better view during low visibility landings. The nose attitude is slightly lower, resulting in a shorter distance from the pilots’ eyes to the first bit of runway that they can see. If they’re looking for 3 centreline lights, for example, it would be a shame if the nearest one was hidden under the nose!
Unless restricted by the AOM or its supplements. The Q400 supplements in the AOM for CAT II and CAT IIIa do not permit a Flap 35 CAT approach. They're restricted to Flaps 15.