Some quests or Diplomatic options are only available to certain Empires or Government types. For instance I believe Autocracy allows you to marry off one of your relatives to another Empire and thus ensure that Empire will never declare war on you. Other such options are also available for each Empire and Government type but you will have to experiment to figure them out.

Also various government types offer advantages in different areas. For example Democratic and Republican Governments tend to offer better peaceful annexation features and more money from trade and taxes where as Autocracy and the others appear to give you better conquest options and unit restoration features.

Having played the Democratic and Autocratic games, I can say the biggest difference is that Democratic governments have to buy everything but have the cash to do it. Though they often run short on other resources. Autocratic Governments usually have plenty of people for the army and other resources but run short on gold. Also some of the more advanced tools for taking over other countries without a fight such as Resistance cells appear to cost more if your an Autocracy.

Written by giskard