Sadly many of us have experienced the disappointment of breaking your screen on your beloved iPhone and immediately start shopping around to find the cheapest option for getting it fixed. However, this is not always a good idea and the best option, although more costly is to go to Apple or an authorised reseller, and here is why (by the way this information is from my own investigations when having to get a screen fixed and Apple have not encouraged me to say any of this).
1. When you drop the iPhone, a common way to end up with a shattered screen, it may have done more damage inside, the phone may continue to work ok but something may be lurking. It could be the SIM card tray which may only be a problem when swapping or removing a SIM card or something else.
2. Apple use genuine Apple parts to replace the screen so you know it fits better. So although the phone is not waterproof there is a better seal if done by Apple to stop little bits of water and dirt from getting in.
Apple replaces your iPhone, iPad or iPod touch with a fully refurbished one, so no waiting around for it to be replaced. They will run though a checklist with you and help set it up.
You do need backup your phone before taking it to Apple.
You may not be bothered and willing to take the chance as the bottom line is often the cost and willing to chance that something else may not be broken, but if you are wondering why it costs more at Apple this may have helped.