Plugins in plugin.ServeConfig is confused

type ServeConfig struct {
HandshakeConfig
TLSProvider func() (*tls.Config, error)
Plugins PluginSet
VersionedPlugins map[int]PluginSet
GRPCServer func([]grpc.ServerOption) *grpc.Server
Logger hclog.Logger
Test *ServeTestConfig
}
It is ServeConfig struct.The Plugins is a type of PluginSet,which is constructed of map[string]Plugins.
But in common,a plugin process only contains one function.So the Plugins(map[string]Plugin) only has one string-Plugin pair in most time.Plz tell me why it is designed like this?

Sincerely thanks