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

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

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