1. To raise partner’s suit (eg 1♥-Pass- 2♥ or 1♦-Pass-1♠-Pass-3♠).
2. If you have a worthwhile holding in partner’s suit, this is support.
Support can be primary (usually four cards) or secondary (usually three cards). When evaluating support, length is more important than strength. Supporting your partner’s suit, especially if it is a major, is normally a good idea.
One of the key objectives of bidding is to establish the denomination in which to play; supporting partner facilitates this. Supporting partner’s second suit nearly always shows four-card support.