Walt Disney World was Walt’s opportunity to improve upon the design and logistics of Disneyland, using his experience and the much greater land area in Florida. One of those improvements was building tunnels under the Magic Kingdom to facilitate the backstage activities that park guests don’t need to see.
When construction began on the Magic Kingdom in Orlando, the first layer of the park that was built was 392,040 square feet of “underground” tunnels, known as utilidors. Fun fact: They’re not actually underground. The “basement” level of the park is actually at ground level, and the part of the park visitors experience is the second floor. Cast members access the utilidors via staircases positioned at key areas in the park.
It’s a matter of maintaining the illusion of magic. The "utilidors" are used for all kinds of things, which you can read about at mental_floss.
(Image credit: Solarius.com)