Lubuntu is a lightweight version of Ubuntu which is a Linux distribution. And GNU/Linux is an operating system which actually is most widely used e.g Android. Check this out;
Android is Based on Linux, But What Does That Means
https://www.howtogeek.com/189036/android-is-based-on-linux-but-what-does-that-mean/
There could be many reasons someone would use linux when presumably Windows and Mac work just fine. My reason is that it's free;no licensing, I get full control over my system, I know what's being installed and what not, it is lightweight for older and low specs laptops; no slow downs, crashes, etc.