First and foremost it is to share with others quality Joni Mitchell music that others have been lucky enough to obtain.
Trees also serve the purpose of helping new tape collectors to get their collections going.
Another motivation for running trees is to try to discourage people from feeling the need to partake of the large commerical bootleg market out there. Commercial bootleg producers are largely seen to be disrespectful of an artist's rights by cashing in on the artist's music without any consideration for the artist. The first step in the tree process is finding the seed tape. A seed tape is the tape that will ultimately be used to make everyone's copy. These days recording equipment has gotten so small that almost every show gets taped one way or another.