All answers are right !
He likes it : yes, I like it.

I do it on all forums.
He doesn't know that his name is shown : Imagine that the forum is hacked or destroyed... on a forum... it happened that some technical members had disappeared. And many messages were consequently without author... (signature can help us)
He wants someone to wonder that : yes !

I did it ! I made someone wonder about it ! Hip Hip Hip Hourra !
Other(Shokin, that's for you, tell me) :
- For confidence and safety : if someone found my password he could put any message... when he does not sign, that is not me. But... this person can sign at my place... so... my reason is not a good reason.
- I began... and it became an habit...
- I want you to remember me.
- If someone copy-stamp my message somewhere else, he/she won't forget the author.
On many forums... you can add your automatical signature (on englishclub, there was a time when you could). And I usually put in my automatical signature a sentence, a proverb, about making people conscious... for example :
- Human know-how is more ecological than machines.
- Daily good or bad actions/acts have more consequences than occasionnal actions/acts.
- Le respect est un aspect devenu circonspect. Il est désormais suspect.
- Le verbe faire est un verbe qui se perd. Tout le reste n'est que chimère.
- Agir partout au quotidien pour la nature.
...
Now... on this forum I cannot.
- Each message is like a letter (to your lover or to your boss

).
Shokin