Short answers, pulled from the story.
The first SWAT unit was established by the Philadelphia Police Department in 1964 as a 100-man specialized unit in response to an increase in bank robberies. The Los Angeles Police Department followed in 1967 with its own Special Weapons and Tactics unit, which became the national model.
Officer John Nelson conceived the idea for the LAPD unit, and Inspector Daryl Gates approved and formed it from volunteer officers. The first LAPD SWAT unit consisted of fifteen teams of four men each, totaling sixty officers, organized as D Platoon in the Metro division.
The first significant LAPD SWAT deployment took place on the 9th of December 1969, when officers attempted to serve arrest warrants on the Black Panthers at their Los Angeles headquarters at 41st and Central. The resulting standoff lasted four hours, during which over 5,000 rounds were exchanged, and four Panthers and four officers were injured.
The Columbine High School massacre on the 20th of April 1999, revealed that the standard practice of setting a perimeter and waiting for SWAT cost lives. In response, departments began training and arming regular patrol officers to respond immediately to active shooter situations, rather than waiting for specialized units to arrive.
By 2005, SWAT teams were deployed approximately 50,000 times annually in the United States. That figure rose to nearly 80,000 times a year by 2015. An ACLU study found that just under 80% of deployments were to serve arrest warrants, most often drug-related warrants in private homes.
A study by Professor Jimmy J. Williams and Professor David Westall found no significant difference in the frequency of use of force between SWAT and non-SWAT officers when responding to similar situations. This finding complicated both critics who viewed SWAT as inherently more aggressive and defenders who argued specialized units reduced force overall.