Not all teeth can or should be saved, sometimes there is no other option. Choices comes later, when you consider what should be done to replace your tooth and this is where you want as many options as possible.
The range of options for future replacement of your missing tooth is greatest where there is bone available where the tooth was taken out. Unfortunately, as soon as your tooth is removed you also start to lose the bone that supported the tooth, unless something is done to maintain it.
A bone graft allows you to maintain as much supporting bone as possible and gives you the opportunity to at least have a gum shape that helps your replacement tooth look real. Even if you would only consider replacing your tooth with a denture, the grafted site will allow a denture to be more retentive than it would otherwise have been.
In an ideal world, a graft would be placed each time a tooth was taken out.