Why is there a compression level for PNG images?

[pstein]

When I save a picture as .PNG then the options how a compression level for .PNG format.

Why?

I thought PNG is lossless and hence has NO more-or-less compression but the best one.
It is the first time that I have seen such a level for PNG in all the image editor prgms.

Peter

[XnTriq]

PNG compression is able to considerable reduce the file size of an image without any loss of quality. While the jpg compression is quite lossy, in png compression there is neither a change of colours nor a reduction of colour depth. The number of colours exactly remains the same.

Ten steps of compression can be used for png images. Below is shown, how an image with maximum amount[1] of possible colours is compressed from a file size of about 197kB, which is roughly the same as the corresponding bmp file size to 678 bytes, which is just 3.4‰ of the original size. Notice that already the first compression rate reduces file size to 1.8kB, i.e. 1% of its original size.

[Danny]

Yeah, it doesn't really make sense since the program should find out by itself which level to choose for best results.

Just crank it up to max and never look back i'd say.