Depends on how you intend using your phone. For example, if you are a self-employed delivery driver, and some deliveries require a signature, the pen will save you having to wipe off grubby finger smudges from the screen, with the added bonus of having a more legible signature. Likewise, if you are in an office environment, where you need to quickly scribble down notes.
If you use your phone for making calls, taking photos, etc (basically, anything that doesn't require a pen), then it is a waste of time, and money.