Spirits want the ability to communicate, share,
and be part of something. Spirits do that by coming into our lives themselves
or by being invoked or conjured. Once they have come to us they can stay of
their own will, attach themselves to a person or vessel, or be bound
temporarily or permanently to a person or vessel.
No one wants to think that after they have gone there will be no purpose or usefulness to their lives. Many have reported being "haunted" by loved ones or ancestors, these reports coming even from those who are not typical subscribers to the paranormal. This is no different in the aspect of spirits from our own realm & other realms; they too wish to be of purpose & assistance. Some are set on accomplishing help with a particular human whereas others roam unbound until there is a specific need for their guidance, support, help, friendship, companionship, etc.
They have the capability of being on our realm as well as the spiritual realm, and in the case of those who originated from the astral realm or spiritual realm some have the capability of walking all 3 planes. This accounts for why sometimes their presence is more strongly felt than others. They aren't always sitting around waiting for something to happen; they are capable of coming & going to and from you to their own realm on their own.