it confirms that your GPS chartplotter position is correct. Some of the charts up there warn you that they may be off by up to 0.3 miles. By overlaying the radar image on the chart plot you get confirmation that all is good.

Having spent the summer up there, I found the chartplotter and depth sounder FAR more useful for navigation, even in pea soup, than the radar. The radar (newish Raymarine 2KW) would not reliably pick out buoys, small boats, or low lying land features. If it was all I had, I'd use it. But I played with it quite a bit, often in perfect visibility just to see what I was seeing. It left me lacking confidence in its dependability. Buoys with reflectors on them could sometimes be seen at a couple of miles, sometimes not seen even at 300 yards. Small boats might show up pretty good, or might not show up at all. With a seaway, distinguishing sea clutter from a small boat was often difficult even when you knew exactly where to look. Ships were nearly always visible though, as were higher land features.

Fortunately, the traffic was quite sparse. We rarely saw other boats. Real "traffic" was limited to a few areas: the tour boats around Tadoussac and Gaspe, lobster boats and sailboats around Shediac, ferries and ships around Port Aux Basque and Halifax, and shipping around Cape Sable. Large areas were more or less deserted and we would go days without seeing another boat. The lobster pots in the Northumberland Strait and Maine were frankly a greater hazard to navigation, and the don't show up on radar!