First, the satellites aren't located only over the equator, they're over most parts of the earth in low orbit. As the platform (boat) you are on keeps changing direction, especially at anchor, you're more likely to pick up a satellite by the dish pointing straight up. With in the antenna, there is some ability for the antenna to focus on a particular satellite, and while underway, to change satellites. So, from what I understand, looking straight up gives the antenna more satellites to choose from, and in remote areas with less satellites, to be able to electronically point at the available one.
Second, there has to be some energy savings as you swing at anchor, not trying to keep tilting at a fleeting satellite as you swing.
Lastly, when in channels with steep hills or trees, you're much less likely to be obstructed with the antenna pointing straight up as opposed to being angled. This was especially true for me when in the narrow channels with high cliffs of Isle Royale National Park and also when traveling through the Dismal Swamp canal.
Ted
