Spirits have many different ways of surviving while not knowing where to go.
They may need to:
Being a "good" spirit lost in our realm is no treat for them. They have fear, confusion and misunderstanding of what is happening to them.
They may be people that we loved or people that were friends. They may also be people that we have never met, but we have entered their territory, a house for example, and need you to provide their energy to continue to exist. This is the form of attachment that spirits do most of the time.
But, "bad" spirits need to survive to torment a person or people. They need negative energy or the energy of a person to thrive on to keep going. They may be satanic, demonic or just plain harmful and may have already existed at the location they are at, or have attached themselves to a person to keep doing harm to them.
Thus, it is very important to recognize spirits that may be around us and help them to be where they want or need to go to be happy again. Or, need to be removed because they are doing so much harm.