Because no single format is the best for everything, There are differences in filesize, quality, method of image creation, color space, transparency, animation, compression techniques, compatibility, etc.
For a company logo, it would be a terrible idea to use jpeg, a vector format like svg would be far better.
For a photograph, you might use png or jpeg, depending on your quality vs size tradeoff.
For a loading animation, you might use gif.
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