Answer:
Originally, they were developed to treat mental illnesses like schizophrenia or bipolar disorder. Whether they were effective or not is a different question. Eventually they grew to be used for things like chronic pain and the ending of epileptic seizures. For example, the case of "Patient HM" Milner was someone who had a lobotomy to end his debilatating epileptic seizures, and the surgery worked! But, he lost the ability to make long-term memories but did have somewhat of a function for very short-term memory. In other words, he could not make new memories.
Nowadays lobotomies are considered barbaric and not something practiced due to cases like Patient HM.
