The reason why you would want to optimise your images (reduce the image file size) is to find the best balance between Image Quality and Image Size.
Using high resolution images usually translates into very large image files. Add multiple large images into your issue and you end up with a magazine app that takes long to download, which is not the best user experience you are looking for.
The way to combat this is to optimise your images before you use them in your issue design. This is done by compressing the images using compression tools included in your photo editing software or simply using online free compressor tools, such as the following 3 examples.
These tools reduce the image file size without compromising the quality of your image, and in turn prevents unnecessary file 'bloating' when these images are added in the design phase of your issue.