How else can you store the state of the object then?
When we say session we mean a tmp file on the file system.
If your user only has one attribute "name" which is "Kyle" then arguably, perhaps it does not make a lot of sense, you might as well store a json encoded array or just $_SESSION['name'] variable.
But if the user object is storing a lot of data and perhaps even state, then serialize/deserialize provides a good framework/contract to store/get that data without having to "know" that the key you want is "name" as you attempt to unbundle it.
Maybe there are other checks that can be done to make sure you are unbundling the correct type of object.