Applying drywall tape helps bond together adjacent sheets of drywall.
Doing this creates continuity and helps reduce any motion or cracking. Joint compound can turn into powder if you neglect to apply tape to shore it up.
Typically used in bathrooms, basements or any other rooms in which humidity is probably high.
It helps hold the joint compound in place. Without the tape to support it, the wet joint compound is more likely to sag out of the joints.
