Median palatal suture is a relief area while making upper edentulous impression  because:
Maxilla

Supporting structures

Hard palate—rugae
Residual alveolar ridge—maxillary tuberosity


Limiting structures 

Labial frenum
Labial vestibule
Buccal frenum
Buccal vestibule
Hamular notch
Fovea palatine
Posterior palatal seal area


Relief area:

Midpalatine suture
Incisive papilla
Torus palatinus


Stress-bearing areas 

Primary
Horizontal slopes of hard palate lateral to
Median sutures
Secondary
Crest of the residual alveolar ridge
Rugae
Maxillary tuberosity
It overlies nerves and vessels
This area is prone for resorption
Overlying mucous membrane is thick
The submucosal layer is extremely thin