Many people initially go to the ER because they are feeling unsafe and want/need help. They are seen by a pdoc or a mental illness crisis person and it is decided that "Yes" this person needs to go IP for their own safety or the safety of others.

The person seeking help is told and may decide that "No" they don't want or need to anymore, Profess how they are just fine and want to just go home, tries to escape the ER, becomes violent trying to harm self or hospital staff , that kinda stuff would result in being "Baker Acted"