Playing a card that provides +buy increases the number of cards you are allowed to buy during your buy phase.
+Buy is an essential component of most engine strategies, since engines can take a long time to build; +buy allows you to acquire engine components more rapidly, thus speeding up the construction of the engine, and once the engine is running it allows you to buy multiple Victory cards at once to make up for the slower start. An easy trap to fall into is that of "overbuilding" an engine in the absence of +Buy: there's no use in building a deck that can regularly generate each turn if you can still only buy one Province with it. In some engines, it is good strategy to wait to buy even a relatively cheap +buy card until the engine is regularly producing lots of each turn, though, if the +buy card doesn't itself help the engine get moving—"paying for Herbalist".
In a Big Money strategy, +buy is usually less useful, since on most turns in most Big Money decks you just want to buy the most expensive card you can, and you'll rarely have a turn with enough to buy both a Province and something else useful. The opportunity cost of getting a card just for its +buy is usually too great in these decks.
Gainers and remodelers are sometimes referred to as "virtual +buy", since they provide other ways to gain multiple cards per turn. Gainers and remodelers don't depend on having enough money to spend to buy additional cards as +buy cards do, but they are usually less flexible in other ways.
Some cards that give +buy have it as part of their core function, such as Woodcutter: virtually the only reason to buy Woodcutter is if you want +buy. In some cases, a card offers +buy as a feature that synergizes with its core function: Goons offers you a + reward per card you buy, and gives +buy to make the reward go farther. Other cards have +buy merely as an extra vanilla bonus that keeps the card competitive with other cards at the same cost: Margrave is a useful card even in Big Money if you never use its +buy, but the fact that it has +buy compensates for the fact that its attack is not very strong and makes it easier to include in an engine.
 Cards that give +buy
Cards in italics have been removed.
 Terminal +Coin/Cost reduction
- Trade Route
- Horse Traders
- Nomad Camp
- Spice Merchant
- Merchant Guild
- Bridge Troll
- Wine Merchant
- Farmers' Market
 Treasures and disappearing money
- Pawn can provide +Buy in combination with either +Action, +Card, or + .
- Courtier offers +Buy as one of several choices.
- Tactician gives +Buy on the next turn only.
- Ruined Market provides only +Buy; it's occasionally claimed that this is the one Ruins card you might intentionally gain in order to build an engine.
- Sir Martin has +2 buys as his vanilla bonus in addition to the Knight attack.
- Travelling Fair is a sort of "+Buy village", turning 1 Buy into 2, while also letting you top-deck cards you gain.
- Seaway gives +Buy to an Action card pile in the Supply.
- Forum gives +Buy when bought.
- Engine strategies in general
- Alt-VP rushes and deck-bloating strategies such as Gardens, Silk Road, Vineyard, and Philosopher's Stone
- Highway and Quarry