The build data table activity will assign a datatable to a variable with exactly what you setup (columns and rows if desired). Each time it executes it would overwrite the variable in the output property with the datatable you setup in the Build Data Table wizard.
You can use the add row activity to insert a single row of data to the datatable once it has been built (or created using other means).
If you were to execute the build activity again (assigned to the same variable) after the add row activity you would not see the row you added in the datatable anymore because the variable would have been reset to the configuration in the build activity.
Hope that helps provide some clarity to these activities.