To remove the dead persons identity as a human being...
We can agree that most of us recognize each other by our faces...
Beheading is a demoralizing tactic to remove the dead persons identity and therefore their recognition and worth as a known individual...it also shows that they think the people they do this too are faceless, nothing, without an identity you are worthless.
These savages get their rocks off from torturing and beheadings
The Muslim hard core Jihadists behead their enemies to strike terror into the hearts of their enemies. To demoralize their enemies. Demoralize him and he's easier to defeat and conquer.