You should submit to directories for traffic and traffic only. Only thing that is confusing is that you shouldn't expect direct traffic(you still may get some), but traffic from SE as result. For example if your website name is "adventures of green monkey dot com", I am sure you will be ranked no.1 in G for your site title and submitting to directories is waste of time and money. If you go after more competitive terms it is much different story.
In theory, but in practice it's much different. A niche directory is likely to be promoted more heavily to that certain niche, and the listings should be approved by someone who has more enthusiasm for the topic, than for a broader set of subjects.