The DCT is a cool primitive. By extracting the low frequency coefficients, you can get a compact blurry representation of an image. This is used by preload thumbnail algorithms like blurhash and thumbhash. It's also used by some image watermarking techniques to target changes to a detail level that will be less affected by scaling or re-encoding.
I made a notebook a few years back which lets you play with / filter the DCT coefficients of an image: https://observablehq.com/d/167d8f3368a6d602