In such cases, since a tax revenue stream exists or will be created by development on the property, bonds can be issued to finance infrastructure and services. In that manner the developer or property owners can avoid having to front the costs. The bond holders do that instead, expecting to be paid back from future tax revenue.
The Reedy Creek Improvement District went further. The District board was effectively under Disney's control, had broad local government style planning and permitting powers, and extended over large areas not owned by Disney. This made Disney exempt from having to get planning and permits from anyone other than their captive district board. In effect, Disney World had governmental powers.
The new district board appointed by DeSantis is not separate from but is the successor to the original Disney controlled board. In that manner, Disney now has to answer to DeSantis appointees instead of to no one but itself.
Why not just a county board? More local control, in this case Orange and Osceola.