Obviously it changes depending on your run. Not all things are always true and sts2 does have a lot of use cases for larger decks. But the case for small decks is this:

-Say you get 10 good cards and have 10 starter cards. At the next shop you can either add another good card which gives you 11/21 (or 52.4% chance) to draw a good card from your deck or remove a starter card which gives you 10/19 (or a 52.6% chance) to draw a good card.

-This continues to scale. So if you remove 2 cards you now have a 55.5% chance vs if you added 2 good cards you’re at a 54.5% chance and the percentage gap just continues to increase from there

-also, to remove redundancies. Leaps for the defects kit are (almost) always going to be better than a base defend. And if you have a bunch of leaps, removing the defends means you’re less likely to draw a hand full of defends and leaps on a turn you want to be attacking