Bernd 05/19/2017 (Fri) 17:43:12 No. 6839 del
+15 opinion boost, and more likely to keep alliance (I think +10 reasons).
But it's most useful because you can claim throne of countries with same dynasty and royal marriage. It gets you in a succession war on the defending side so if you have strong diplomacy (like I do) it can help you both inherit strong nations and a way to passively-aggressively go to war with strong rival countries (which will challenge your claim).