Same feature was discussed in above topic.
Take my usecase for example. I am new to Discourse and actively follow meta. Currently I have more than 60 or 70 bookmarks i guess. These bookmarks include topics related to plugins, multisite, how-to’s, features etc. In future If I have some doubt in multisite configuration, I can just goto that bookmark label and dig through the topics. If not I have to search in my entire bookmarks for related topics(more noice).
I am facing similar issue on Quora now. I have been bookmarking interesting answers on it. After sometime now when I want to dig through a specific genre of answers, I am crippled. I am using the search feature these days, instead of going through all my bookmarks.
With labels, user gets more freedom to bookmark and lets him organise his relevant content on the forum.