Performance is not what concerns me, it's design. To what end is it useful to have distinctions between libraries and applications? If I have a JSON parsing lib

Performance is not what concerns me, it's design. To what end is it useful to have distinctions between libraries and applications? If I have a JSON parsing library, it might be nice to run one-off scripts that grab a certain value and pipe it somewhere else. It's also useful to deserialise in complex manners inside an app of my design. My belief is that programs whether they are intended to or not usually end up being composed with other programs so on that basis, having the distinction of bin/lib doesn't serve a useful end to me at least.