If you're building a scene or experimenting with lighting you and set up your own lights, you probably have to zoom in and out on the scene repeatedly: move your props, edit the light properties, zoom back in into the scene to see how the setup looks like or what the lighting does, zoom back out again to make more adjustments... If you have done this, you will know that the procedure is daunting. If you set a lookmark for the perfect angle for the shot, you can zoom out as often as you want and edit what you need to modify. Zooming back into your scene to your perfect angle is just a mouseclick away.
If you're working on a large creation, it might be useful to look at it from different perspectives, e.g. a bird perspective from above, side views, etc. If you are working on details, you need to zoom in to see what you are doing. To get an overall view you need to zoom back out again. Lookmarks are a means to collect all the views you need and switch between them.
Say, you are looking for a dress. You are in a store which has many beautiful dresses and you can't decide which one to get. Set a lookmark on each vendor board or model wearing a dress you like. When you're done, you can cycle through the lookmarks in Spyglass and have a fast comparison of all your picks without walking through the store again.
If a store has lucky chairs at different locations, set a lookmark on each of them. When you're shopping at the store you can quickly cycle through the lookmarks to check whether your letter shows up.
Midnight mania boards have become quite popular. If you are in a mall where there are a couple of boards probably you won't remember each location if you go there next time. Just set a lookmark on each board you see, and you will have them at your finger tips the next time you're in the region. Even register for the prices without teleporting from board to board.
By setting a lookmark in your skybox or at your club you can always quickly go there with your camera, and you know what it is going on.