The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
Star Chart’s effect looks innocent at first glance, but the consistency it provides is extremely valuable in the first few turns of the game, to the point where it’s difficult to imagine situations where it’s not worth it to get Star Chart within that time frame. Along with that, it continues to be valuable throughout the entire game.
The most common situation for Star Chart is to open with it, ensuring that you see your other opening buy on turn 3. This makes opening with duration and reserve cards much better, and shores up the chance that a critical card misses the shuffle, which is frequently more valuable than buying another card in the opening. There are a few exceptions to this, the most notable of them being: a 5/2 or 2/5 opening, or maybe if hitting $5 on both of turns 3 and 4 is appealing. Even in these cases, you are likely to want to pick up Star Chart soon.
Given that you have this extra piece of consistency from early on, you can build your deck more aggressively. The need to overdraw just for consistency’s sake is much less, and generally you can prioritize payload a little higher with Star Chart around.