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.
Silk Merchant provides a little bit of everything you want at the start of the game. It increases your payload with the on-gain coffer (which is really, really good at hitting early price points. Opening SM and silver, you’re likely to be able to hit 5 twice, or up to 7 once, on turns 3-4). It increases your deck control with the draw, and the Villager. The villagers it gives helps you pick them up without much fear, which you’d normally otherwise have with a terminal, even though it’s also a drawing card. Extra buys are also something you want to pick up fairly often, and on Silk Merchant, you’re getting them on a card which already otherwise furthers your game plan, making the cost of getting that +buy quite small.
Having said this, you can’t usually build a deck just from these – they are terminal draw, and if you spam these and only these, you don’t end up with much of a deck at all – you eventually won’t have enough villagers to play them all, and you don’t have much lasting payload. So you want to use the card more as a role player, combining with things like villages and economy in order to maximize your deck’s effectiveness.
The on-trash effect is also a nice bonus and is usually quite strong wherever there’s something that gives you benefit for trashing a card (especially one which cares about the card’s cost).