Although most images are non-linear, sometimes you should leave them that way.
Knowing how and when to deal with colour spaces in your textures files is crucial to a realistic rendering workflow.
In short: since monitors are usually non-linear devices, image files are generally stored with the inverse non-linearity baked in (the general example of which is sRGB). Since rendering calculations work in a linear space, you must convert your images back to the linear space before using them otherwise your renders will not be accurate.
While it is key that you linearize your colour textures (e.g. diffuse, specular colour, etc.), you must be very thoughtful of doing the same with your data textures (e.g. displacement, bump, etc.).
For example, lets look at a displacement:
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