Originally Posted by
Kitten Champion
Siegfried's built to kill dragons (also every dragon-trait Servant like Liz and the many, many Artorias) and dragons are both prolific, strong, and very rarely Archers in events and main quests. He is really quite mediocre outside of that, but it is definitely a nice niche to have to go against any of those giant dragons, wyverns, or just fight Artoria/Liz Lancer whenever they appear.
So, having him around is cool, but I wouldn't be levelling mine now specifically if it weren't for the event.
Edit: Also, he has the Dragon trait as well, so he benefits from Jalter's extra Charisma buff. Which is always nice.