to add to Bo's point, locking threads will prevent heated arguments [against the rules] or stop roleplaying and fabrications. they typically lock topics instead of deleting them, because it's easier [so i'm told] but the topic normally contains information that can help others, so instead of deleting the whole thing, they'll leave it just for others to read the responses to. granted if there's an argument, or inappropriate language it will be deleted.
i have seen a lot more locked threads recently, maybe a lot of people are arguing, or perhaps the mods have more free time to monitor the forums more. [i think it's the first option though]