You can use it on your less-pricey embedded board or salvaged computer.
Don't want something in LXDE, or 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 a language LXDE does not cover, or want to help others to adopt LXDE?