The cape cod canal, or Boston harbor where you're going to be in a restricted channel with big ships and ferries, it's really nice to know what's coming around the bend before you see it (especially in reduced vis). It's a nice supplement to radar. In a crossing situation it's also reassuring to be able to see an on-screen closest point of approach calculated.

You may find that the http://www.marinetraffic.com info is good enough as a supplement, but my point above was that it can have delays/gaps.