Linked from
The 12 pages that link to Chroma subsampling, each with the reason it gives.
JPEGRelated: JPEG can exploit human sensitivity to brightness detail by reducing color resolution.
Image compressionRelated: It exploits lower human sensitivity to fine color detail in many images.
HDMIRelated: It can lower HDMI bandwidth demands while retaining much of the perceived image detail.
Video coding formatRelated: Many formats reduce color data through subsampling.