People perceive colours differently. In order to achieve perceptual uniformity, the International Commission on Illumination (CIE) stipulates spectral weighting functions. These functions describe how people perceive colours.
They are based on experimentally determined sensitivity curves of the long-wave L-cone (X), medium-wave M-cone (Y) and short-wave S-cone (Z). The colour receptors (cones) on the human retina reside within a 2° arc of the fovea on the so-called macula. Based on this perception, the CIE introduced the standard colorimetric system in 1931. As the receptors for light intensity (rod cells) are also responsible for human colour sensitivity, the system was complemented in 1964 by the 10° field of view.
This is why there are two weighting functions. The one for small fields of view (standard observer 2°) corresponds to the size of a postage stamp at the end of an outstretched arm. The other is for larger fields of view (10°) corresponding to the size of a postcard at the end of an outstretched arm.