What I don’t understand, does the annotation affect the
de/serialization itself
No. Its retention is ‘source’, so it’s discarded after compilation. The bytecode will contain no trace of it. It has no way to influence runtime behaviour (besides possibly compile-time code generation, which does not happen).
Like @Override, it is optional and is supposed to give some compile-time assurance for problems which might otherwise not be caught until runtime.
For example, misspelling serialVersionUID:
@Serial
private static final long seralVersionUID = 123L; // compile-time error, should be 'serialVersionUID'
Or the wrong access modifier
// compile-time error, must be private
@Serial
public void writeObject(java.io.ObjectOutputStream out) throws IOException
Basically, something annotated with this must exactly match the descriptions of the 7 applicable elements mentioned in the JavaDoc (5 methods, 2 fields). If the signature of a method does not match, or the modifiers are wrong, you will catch the problem before serialization fails at runtime.