Not only do I like GMSV for seeing what's near our hotel, I like to use it when we will be in a port that reportedly has easy access to the city. I like to see what's within walking distance, or if there is a shuttle, where does it drop off and pick up and what landmarks will I want to remember. We went on an Alaskan cruise in 2011, and two of the ports were such that we planned to have some free time exploring on our own - in Skagway and in Ketchikan. We were able to walk the streets ahead of time to get an idea of what was there and how far things were (or in these two cases, weren't). Plus it's fun to get a sneak-peek of what the towns look like. Skagway was right out of a movie set.
For our trip this summer, I'm using GMSV particularly for the ports where we will explore on our own, beginning with our days in Barcelona. While we plan to use a hop-on, hop-off (aka HOHO) bus for a day of sightseeing, we know we'll have some time to explore the city near the hotel. Plus we'll want to find where to get some food, so we can plot out the recommendations from others for restaurants, tapas bars, or the fresh food market and see what we might see.