The mission editor has many objects with which to build custom environments, and these can become quite complex. For example, when creating a fenced enclosure each fence section must be painstakingly positioned to line up with the the one previously placed so that there are no obvious gaps.
Many missions use similar environments: a military base, a communications centre, a farm, a village... so why reinvent the wheel each time?
The OFPEC SetPos Templates section provides various categories of ready-made setups which can be copy/pasted into your own mission, saving you hours of development time.
Also remember that you can submit your sophisticated setups for the good of the Community - just post it in the Beta board in the Forum!