Is DBSNMP used only by Oracle EM by default? I have Oracle database and I need to install agent for Oracle EM. It requires credentials for DBSNMP user, but it was lost. Is it safe to change password via SYS user to provide it to agent? 
Is there any other systems in standard Oracle distribution that connects via DBSNMP user? I'm afraid this user can be used somewhere else.
If it matters database version is 12c, agent and OEM are 13c.
dbsnmp is used by the monitoring agent to connect on the database and report any issues to OEM which will decide if the issues require attention and announce oracle and customer, otherwise it will just classify and display as warnings.

You can change the password anytime as long as the agent "knows" it.