As social beings, of course we really need each other, including in terms of attend friend invitations.
A friend needs us for a her happy day because he wants to celebrate with us, maybe with our presence it will make him happy because someone cares, it can also make him feel still expected ...
Although friends who have long separated from us do not know us anymore because of the changes that occur in us, of course with our presence on a happy day we will be able to remember the memories that we have been through together.
Thanks.