Just like Writers, feature types have their own set of parameters that control how that feature type (layer/table) is being written. These parameters are under the Format Parameters tab in the Feature Type dialog:
These parameters apply only to a single feature type, whereas a Writer parameter applies to all feature types.
For example, when writing to a SQL Server database there is a feature type parameter that defines the spatial index to create.
This is a Feature Type parameter because not every table requires a spatial index, and therefore each feature type needs a way to define it.
Note that there is no authentication parameter at the feature type level because authentication applies to the entire database, not the individual tables. Any parameters that apply to the entire database are defined at the writer parameter level.
Writer Feature Type parameters provide a degree of individual control over writing different layers or tables.