While a database is not strictly required to run EventSentry (it will install & run just fine without), it is recommended for most scenarios.
If you want to take advantage of the extensive reporting system of EventSentry, then you will need to install a database and configure the EventSentry agents to collect data (e.g. event, disk space, performance, etc.) in the database.
If you install EventSentry without a database, then you can still utilize all features, but you will only be able to take advantage of the alerting capabilities of them. For example, you will be able to receive selected events by email and be notified when a performance counter exceeds a threshold, but you will not be able to chart a performance counter over a period of time.