^ Exactly. Division and succession to command are apparently unconnected. Line officers can serve in divisions other than command; officers in other divisions can be closer to succeeding to command than officers in the command division. If the "command" division were called something else, say "control division," there might be less confusion on the issue.
As far as the OP question, on a USN warship the Navigation Department is usually one of smallest departments, if not the smallest, with just a couple of officers and a number of enlisted quartermasters. Nonetheless, it is obviously a very important department with immense responsibilities. I would imagine it would be similar on a starship: You may not need them all the time, but when you do need them it would be good to have some specialists with a lot of experience moving big ships safely around. (Of course it could all be automated, but so could most of the functions aboard ship, and Trek has not gone that direction.)