this service is mainly used for jobs, like backing up database ect... if you have an updated version of SQL like 2012 than this service would be disabled...
By default, the SQL Server Agent service is disabled when SQL Server
2012 is installed unless the user explicitly chooses to autostart the
SQL Server Agent uses SQL Server to store job information. Jobs
contain one or more job steps. Each step contains its own task, for
example, backing up a database.
SQL Server Agent can run a job on a schedule, in response to a
specific event, or on demand. For example, if you want to back up all
the company servers every weekday after hours, you can automate this
task. Schedule the backup to run after 22:00 Monday through Friday; if
the backup encounters a problem, SQL Server Agent can record the event
and notify you.
also like to note that sharepoint comes with SQL express version by default and that service is disabled by default. This service would be set to automatic on live (error logging and backup is vital on live) and disabled on test / dev enviroment (no need to waste resources)!