The biggest airplanes first used water for taking off and landing, and only later were similarly sized land airplanes built. Bigger airplanes tend to have a higher wing loading and so require longer runways.
The Caproni Ca.60 was the biggest aircraft of its time (1921) and is still bigger than a Jumbo Jet if you use wing area to measure size. It could only be built as a flying boat.
Caproni Ca.60 (picture source)
Another biggest aircraft of its time was the Dornier Do-X from 1929. On one occasion it carried 169 people into the air, including one stowaway. Its regular capacity of 60 to 100 passengers would only be surpassed in the jet age.
Dornier Do-X in flight (picture source). This one actually flew, but was hampered by the small aircraft engines which were available at its time.
Flying boats were also used on the first transatlantic and transpacific routes, and to connect the parts of the British Empire by air, Imperial Airways used a fleet of floatplanes and flying boats. Routes were strung along lakes as staging points.
Short Empire as operated by Imperial Airways (picture source)
Long runways were only built during the second World War when the weight and speed advantage of land airplanes made them the preferred choice for bombers. With the runways in place, flying boats soon disappeared.