I assume you are asking about the time that Miss Havisham does this early in the book.
If that is what you are asking, I think there are two answers. The first answer is the reason that Miss Havisham actually gives at the time that she invites Pip. What she says is that she wants someone to keep Estella company -- someone young to play with her.
But it seems likely that Miss Havisham also wants someone for Estella to ensnare. She's raising Estella to break men's hearts and get revenge for her own misfortune, right? So I think she wants to give Estella someone whose heart she can break.