There are dozens of reasons why a family or even a single person may decide to adopt a child. Sometimes, the child involved has a biological relationship to the person considering adoption. It's common to adopt nieces, nephews and even grandchildren if your siblings or children cannot handle the pressures of parenting. Other times, someone who can't have children for one reason or another decides that adoption is a great choice for building a family.
Regardless of why you're seeking to adopt a child, you should expect a relatively complex legal process. The state carefully vets people considering adoption, and there is a lot of paperwork that you will need to execute to complete the process.