Typically the Memento pattern is used to implement roll-back/save point support. For example I might want to mark the state of an object at a point in time, do some work and then decide to revert that object back to the point at which is was marked.
The implementation of a Memento pattern could use serialisation, which would involve saving the contents of the object into a byte[] and keeping in memory or writing to disk. When reverting the content of the object would be rebuilt from the serialised copy.
Conversely I could implement a Memento pattern by cloning the object in memory and keeping a reference to the copy and then copying the state back if the object needs reverting. This method doesn't use serialisation.