The purpose of these rules is to keep the gameplay moving back-and-forth. This will be a longer, more design-focused answer, so bear with me.
Each card in Outpost 18 is really two decisions. Because of this dynamic, +Draw openings can generate a hand of 3 or 4 cards very quickly. In Outpost 18, that’s 6-8 potential decisions in a single moment. This can be overwhelming and tough to handle in one step at the end of your turn.
It’s especially taxing for new players, or players who play lighter games.
So instead, when you end your turn, you draw new cards. You can begin to think through your next decision while your opponent is taking their turn. Before you start your turn, you have to discard down to 3, which is the max hand size.
Essentially, I’m just spreading the “Draw / Discard / Hand management” step from TCGs across a longer period to help prevent analysis paralysis.