The systemd unit file documentation suggests that the "Description" field should be specific and meaningful to the end user.

Currently, Mongo's unit file (debian/mongod.service) has the description "High-performance, schema-free document-oriented database", and because systemd considers the description a friendlier version of the unit name, it uses it in log files, like so:

systemd[1]: Started High-performance, schema-free document-oriented database.

Following the example in the systemd docs, maybe something like "MongoDB database server" would make more sense.