For yet another time I was challenged by the same question. What makes it more interesting is that it's one of these few times that the person who asks me this question seem to at least understand the technical merit of what am I doing in the demoscene.
Most people might not understand what a demo is. It's the same as a video on youtube for them. But few other people I met who happen to either study informatics or are computer geeks, still wonder why do I spend my coding abilities in a kind of software that is useless? They seem to understand the hard work behind democoding and they even adore what I am capable of, yet they'd wish that I spent my creativity into producing some usefull applications or games or anything else that is more usefull than demos. They find that it's a loss of talent.
Heh! I laugh at this question because I feel that it's like asking Picasso about the meaning of painting. Some people like to say that demomaking is a new kind of digital art. Usually they also argue about what is art and what's not. There are several definitions yet there is a special characteristic of art that also applies to the question above.
Demos are senseless. But that is art! Creating demos not because they have to be usefull but just because you felt like being creative is the same as composing a music song, writting a poem or painting a picture just for the sake of it. Art is done for the sake of itself! It doesn't need to have a meaning in the sense of what regular people call "a meaning".
There is no answer to their question. Because "usefull" is defined in a way that cannot be understood inside a creative community but only makes a sense in everything we do in real life. Though the greatest thing that happened to me after I was challenged to answer this question is that I loved and understood what I was doing in the demoscene in a greater degree. My reflection upon the inability to find a good answer plus the fact that this "uselessness" is a major characteristic of art that completes itself had as a result that I was fullfilled by a feeling of greater sense about demomaking.
And suddenly I was happy by the thought that I was involved in such a creative community instead of rejecting anything that is not supposed to be making any sense in the real world. Senselessness does make a sense! More sense than real life..