This is especially worthwhile if you are collecting old "classic" stamps. Classic stamps may be considered as those printed prior to 1900 roughly. Obviously, many so-called classics are too rare to hope to obtain anyway (for most of us), so the idea of a spacefiller in such cases is just that - to fill in a space that would otherwise be empty. However, spacefillers can teach you a lot about the stamps of that period, and the fact that they are damaged and not much sought after makes them readily obtainable. Assuming that the spacefiller is in fact genuine, which you can establish fairly quickly, you can use it to test for a particular watermark, you can check the paper colour and texture, you can measure perfs if there are any, and you can examine the printing process on the actual stamp design. This will give you more insight into what the stamp should look like in all respects, and may help you avoid getting stuck with a forgery at some later stage. I have some spacefillers which are actually near perfect (with pinholes and so on) and they have allowed me to make comparisons with other better copies when they turned up.