Reefing is the way to reduce sail area quickly when your boat becomes overpowered. When buying a mainsail, your UK Sailmaker will determine how many rows of reefs are needed. Buying more reefs than necessary adds expense and extra weight to the sail.
Most coastal cruising sailors buy a sail with a single reef because if the wind picks up too much, there are plenty of nearby harbors to pull into. A single deep reef will be used on most cruising boats in winds between 18-25 knots. Reefing in lighter winds can make sailing less intimidating for non-sailing friends.
A second reef is used in winds blowing 22-30 knots—conditions where most day sailors are at the dock. For those who cruise beyond their homeport regularly, a second reef is a must since they will get caught out in strong winds more frequently.
A third reef is only used by sailors who venture offshore.