Detailed Answer
Penile prostheses are primarily used as a surgical treatment for erectile dysfunction (ED) when less invasive treatments—such as medications or injections—are ineffective. These devices enable men to achieve a functional erection for sexual activity, but they are not designed to restore or increase penis length.
From the Guidelines
"Implant a penile prosthesis if other treatments fail or depending upon patient preference. Patients should be fully informed of the benefits and harms associated with the procedure... Do not recommend penile prosthesis implantation, penile disassembly or sliding technique to patients seeking penile lengthening options."