Just like Readers, feature types have their own set of parameters that control how that feature type (layer/table) is being read. These parameters appear under the Format Parameters tab in the Feature Type Properties dialog:
The important thing is that these parameters only apply to a single feature type, whereas a Reader parameter would apply to all feature types.
For example, when reading an Oracle database (here on a Windows Server OS)there is a Feature Type parameter that defines the name of the geometry column to read:
This is a Feature Type parameter because each table might have a different geometry column name, therefore each feature type needs a way to define it. Conversely, there is no password parameter at the feature type level because authentication applies to the entire database, not the individual tables. Those would be Reader parameters.
So feature type parameters provide a degree of individual control over reading different layers or tables.
NB: Not all feature types need a set of parameters, so the Format Parameters tab is not always present.