What is the use of /ppp l2tp-secret (PPP->L2TP Secrets in Winbox). I cannot find any documentation or examples using this.

It's like a password for l2tp protocol! Like a pre-shared key but not a ipsec!
Anybody know were this option set in Windows l2tp client?

There is no setting for Windows. L2TP secret is not used in end customer setups, it is used in LAC/LNS setups