Because it's fun.
My favorite Brainfuck interpreters.
http://brainfuck.tk http://copy.sh/brainfuck/ http://www.iamcal.com/misc/bf_debug/
This is a good IDE for Brainfuck.
http://4mhz.de/bfdev.html
If you are interested, you can contribute to this Brainfuck compiler optimization project.
http://www.nayuki.io/page/optimizing-brainfuck-compiler