1) Why do we need to clean the atty?
Old thickened eliquid coats the mesh and wick, preventing new eliquid from being vaporized properly.
2) The atty metal wick needed to be wet in order not to be burned out?
It needs to be wet to prevent overheating the liquid which remains, which will cause the very unpleasant "burnt atty" taste. The atty parts themselves are not burnt, but only the liquid residue.
3) What happens if we do not clean the atty? Oxidized by the e-liquid?
Vapor production is reduced and airflow through the atty is frequently restricted, producing "a hard draw." The working parts of the atty are resistant to oxidization, so this is not a concern.
4) By using dental mouthwash, soaked overnight not shortened the atty's life?
It is not necessary to soak overnight. One hour or less should be enough, especially if you gently shake the container a few times while the atty are soaking.
You must rinse the attys very well after soaking to remove as much of the mouthwash as possible, but there will still be some taste remaining from it. This taste goes away soon and is not a problem if you use menthol eliquid.
Do not soak attys in alcohol for long periods. The alcohol may dissolve the glue which holds the atty together.