Estella gives Pip some insight into her purpose with this discussion in chapter 44:
“Is it not true,” said I, “that Bentley Drummle is in town here, and pursuing you?”
“It is quite true,” she replied, referring to him with the indifference of utter contempt.
“That you encourage him, and ride out with him, and that he dines with you this very day?”
She seemed a little surprised that I should know it, but again replied, “Quite true.”
“You cannot love him, Estella?”
Her fingers stopped for the first time, as she retorted rather angrily, “What have I told you? Do you still think, in spite of it, that I do not mean what I say?”
“You would never marry him, Estella?”
She looked towards Miss Havisham, and considered for a moment with her work in her hands. Then she said, “Why not tell you the truth? I am going to be married to him.”
Prior to this discussion, Pip and Estella had been talking about her nature and capacity to love. This character Estella may indeed be marrying Drummle as a service to Pip. She does not love Pip like he wants to be loved by her, but she may have the capacity to understand that she could not fake it just to please Pip. Notice, in this discussion about Drummle, she never says she loves Drummle. In fact, Drummle fits for Estella just because he is of high social standing. Pip aspires to be of high social standing, but it is a stretch, and after learning of his benefactor, it still may be further off.
Another direct perspective or interpretation could be that Estella marries Drummle because throughout the novel she has been learning to seek revenge on men for Miss Havisham. Pip has been her life's work. To marry someone else would be the ultimate injury to Pip. Pip points this work out between Havisham and Estella in these words:
Estella, dearest, dearest, Estella, do not let Miss Havisham lead you into this fatal step. Put me aside for ever—you have done so, I well know—but bestow yourself on some worthier person than Drummle. Miss Havisham gives you to him, as the greatest slight and injury that could be done to the many far better men who admire you, and to the few who truly love you. Among those few there may be one who loves you even as dearly, though he has not loved you as long, as I. Take him, and I can bear it better for your sake!
Great Expectations is a great novel to learn life lessons from. Whether the intention of this marriage is to point out that marriage should not be done for convenience or to illustrate Pip's problem of putting all of his effort into one woman, this much is clear: Estella does not love any man, but for social standing will marry a man who did not grow up in a forge.