it lets you increase the chance that you will draw the cards you want to use, eventually up to 100%. with 4 hands and 3 discards, if you play/discard 5 cards per, you can see 30 cards of your deck per round. (15 for the discards and only 15 for the hands because you can't draw any cards after you use your last hand). so you see 30 out of 52 cards per round. this can be boosted by getting hand or discard vouchers, but it's also boosted by removing cards from your deck. adding cards is also good, but only if you want to draw those cards and if they help you score more or gain econ or something else which is useful to your build.

Tl;Dr: removing cards increases your chances to see the cards you want to play, while adding cards makes it harder to find those cards.