When you install the MSSQL database engine, the SSIS execution objects are always installed. This was done to support the maintenance plans and import/export wizards.
The SSIS service does a couple of things. First, it handles verifying the edition of SSIS that is installed. So, if you do not install it and try to run a package that contains objects other than the basic import/export components, you will get an error indicating that you do not have the appropriate edition of SSIS installed.
Next, it handles security for storing and accessing the packages. SSIS uses DCOM to allow access to SSIS packages stored in MSDB or on the file system via Windows integrated security. Take note that it does not have a built in remote execution facility other than the SQL Agent.
There are probably a few things I am leaving out, but I have only had one cup of coffee so far this morning.