Why have I to use a gamma of 2.2 while using a JPEG picture?

During the rendering process, the texture file have a gamma of 2.2. But the plain color do not need this gamma correction, especially if the RGB values are from a spectral caracterisation.

So, Why have I to use a gamma of 2.2 into my texture files ? Why in the CG world we used 2.2 gamma images ? Why don't we use a gamma of 1 for images with real colors ?