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.
Sentinel’s power comes from its ability to be opened with, as well as it being able to trash two cards when played. The fact that it doesn’t trash cards from hand helps you have some buying power, but the normal downside of trashers that trash from your deck is mitigated with Sentinel because you get to select from five cards to trash, so it’s likely that you’ll find two cards you’re happy to trash.