Does any one know why this jacket has a zipper for what appears to be a liner or inner jacket?

Tech Specs:

Material:

[Shell] polyester; [Lining] mesh and polar fleece 

Insulation:

N / A 

Waterproof Rating:

10K 

Breathable Rating:

20000g 

Core Venting:

Chest vents 

Pockets:

2 Hand, 1 inner goggle, 1 inner MP3 

Seam Taped:

Yes, fully 

Powder Skirt:

Yes 

Hood:

Yes, removable 

Zip-in Compatibility:

No 

Recommended Use:

Skiing, snowboarding 

Country of Origin:

China