SWAT duties include:

• Hostage rescue.
• Crime suppression.
• Perimeter security against snipers for visiting dignitaries.
• Providing superior assault firepower in certain situations, e.g. barricaded suspects.
• Rescuing officers and citizens captured or endangered by gunfire.
• Countering terrorist operations in cities.
• Resolving high-risk situations with a minimum loss of life, injury, or property damage.
• Resolving situations involving barricaded subjects (specifically covered by a Hostage Barricade Team).
• Stabilizing situations involving high-risk suicidal subjects.
• Providing assistance on drug raids, arrest warrants, and search warrants.
• Providing additional security at special events.
• Stabilizing dangerous situations dealing with violent criminals (such as rapists, serial killers or gangs).
• Armed patrols

Source: Wikipedia