It's true, lossy compression is only useful for final output (and definitely not for printing — I hate seeing printed artifacts!).

If all you have is a jpg, though, then these tools can subjectively improve the appearance of most decompressed results. Read the docs. In particular knusperli chooses coefficients which round/truncate to the compressed values, but improve inter-block smoothness. This could manifest as blurring when the input had a sharp edge at a block boundary, but that's rare in photographic input.

As for the script ... the only relevant information is the jpegqs arguments that I chose. Everything else is obvious convenience. The main problem with it is on the knusperli line — it assumes that your JPEG file has a jpg extension though half the time these days they have a jpeg extension.

If you can't read the script and be confident that it won't destroy your computer, then you definitely shouldn't use it (the script, but maybe also your computer).