There are a few reasons. First, while it may be easy enough to measure the width of your window and the height of your curtain, often times it’s more difficult to get an accurate gauge of the depth at which you want your curtain to hang. This is why some DIYers prefer to use wire hangers instead of rod-style hangers—they can be adjusted relatively easily depending on how deep into the wall you want your curtains tacked down.
Second, curtain rods tend to be bulky and awkward to maneuver. Unless you have a lot of space in which to string them all together, or a way to suspend them from above (like using regular hooks), it just doesn’t make sense to install one in most cases. Wire hangers are perfect for both situations: they can be mounted anywhere on the wall, and they don’t require any special hardware or installation tricks.