I have often thought that marriage and vassal obligations arising out of marriage, or related to it, tend to be under-utilised in some games. As Robert points out, it can be a great tool for binding one realm to another, a cause of war, or a very handy DM not-so-random event.
One of the greatest benefits can be for weaker, vulnerable realms stuck between two larger, antagonistic realms. Robert no doubt knows more about this than I do, but it's my understanding that if a landed regent has two liege-lords, it was generally accepted that one of those liege lords couldn't expect or order their vassal to act against their other liege lord. Take Robert's Roesone as a hypothetical example: stuck between a Diemed and Osoerde who dislike each other and are planning war, Roesone swears fealty to both Diemed and Osoerde. Then, neither Diemed nor Osoerde could expect or order Roesone to march with them against the other realm - thereby sparing Roesone significant looses and costs. Of course, such would be a very delicate arrangement for Roesone that would require very careful management.
I've also used this in gamjes I've run. One example that springs to mind involved Elinie and Osoerde. Elinie hosted a grand fair and tournament and, while at the fair, a younger son of an Osoerdean family met a young girl from an Elinien family (who, being an only child, stood to inherit her family's manor and lands near the Osoerdean border). Certain feelings developed between them over the following months as they exchanged flowery letters across the border until they reached the point that they decided they couldn't life without each other. They petitioned their families (who weren't keen) and their liege lords, who refused permission for them to marry - Elinie/Osoerde relations not being terribly good.
The young couple decided to run off together, which is where things really got tense. The Eliniens decided that the evil Osoerdeans had kidnapped the girl in order to acquire lands in southern Elinie. The Patriarch and Jaison Raenech exchanged some harsh words and were on the brink of war before the Points East Traders completed an espionage action and discovered that the young lovers were living together in the Free City of Endier.
They were both disowned by their families. I think from memory that Kalien found out around this time and he stepped in and quietly offered the girl a position in his court and a small manor to the young man to manage, extracting an oath of vassalage from her and her husband. It was a long term gambit from Kalien, because the Elinien family now had no natural heir and Kalien thought he'd cheaply buy a stake/opportunity in inheriting their lands through the girl at a later date.