My semi-educated guess is that you are running the Oracle Data Gatherer/Intelligent Agent (part of the OEM Tuning pack). The dbsnmp process (started and stopped by lsnrctl on Unix) is what collects database metrics and writes them to your OEM repository. It needs a username so that it can connect to the database to gather/write the metrics data.
Yes macswell is right, Oem will create DBSNMP USER. Even if you drop the user also agian it will create when you connected from OEM. The funny thing is it will use system tablespace for DEFAULT and Temp tablespaces. No problem with that user. You can leave him on his way.