You can use it on your less-pricey embedded board or salvaged computer.
Don't want something in LXDE, or you don't want to use LXDE but only part of it? You can choose what you like.
Featuring the interface and shortcuts you can easily get used to.
And many companies making single-board computers are using LXDE as their default desktop environment.
Do you program in C or Vala, use the language LXDE does not cover, or you want to help others to adopt LXDE?