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