1. When one greets his Muslim brother with salaam, he benefits him in three ways:
1) He makes duaa for him that Allah Ta’ala keep him in peace. Salaam is a greeting of peace.
2) He reminds him that Allah Ta’ala is with him and watching him at all times. ‘Salaam’ is the name of Allah Ta’ala.
3) He informs him that when he has greeted him with the greeting of peace, he is certainly his well-wisher and he will not harm him in any way (neither verbally, physically or in any other way).
2. It is waajib to reply to the salaam that is written in a letter/SMS/Email etc. The reply may be given in writing or verbally.