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5. Has to do with image compression

If you consider an uncompressed image format like tiff or bmp, there is no compression going on(generally speaking). Jpg and most other image formats involve some level of compression which varies depending on the algorithm used. Some of these compression algorithms are lossless which means every pixel that went in along with all of its associated data, comes back out with complete fidelity at the end. However most involve some level of data loss as this improves the size they may be reduced.

For the most part, what you see on your computer screen isn't going to be all that different. However, if you want to print your image sometime down the road or edit it, especially with crops and enlargements, every pixel matters.