A bit of history from Wikipedia:
In October 1957, Screen Gems released a bundle of old Universal horror movies to syndicated television, naming the collection "Shock!". They encouraged the use of hosts for the broadcasts. This is why many of the early programs were called "Shock Theater". Viewers loved the package, as well as the concept, and ratings soared. A "Son of Shock!" package was released in 1958.
Creature Features was another film package that was released in the early 1960s and added to in the 1970s. The films in this package ranged from horror and science-fiction films of the 1950s, British horror films of the 1960s, and the Japanese "giant monster" movies of the 1960s, and 1970s. This package also included an uncut print of Night of the Living Dead.
The first television horror host is generally accepted to be Vampira. The Vampira Show featured mostly low budget suspense films, as few horror films had yet been released for television broadcast. Despite its short 1954-1955 run, The Vampira Show set the standard format for horror host shows to follow.
Hosts were often plucked from the ranks of the studio staff. In the days of live television, it was not uncommon for the weather man or booth announcer to finish a nightly news broadcast and race madly to another part of the soundstage for a quick costume change to present the evening's monster tale.
While a few early hosts like Zacherley and Vampira became the icons of this nationwide movement, most hosts were locals. The impact of these friendly revenants on their young fans cannot be overestimated. The earliest hosts are still remembered with great affection today.
It's also worth noting that among the kids staying up late to watch Shock Theater were the aspiring film makers like Spielberg and Lucas who would be greatly influenced by what they saw when they went on to largely invent the modern blockbuster era.