Many iPhone users, and other smartphone users alike, charge their mobile devices to 100% as it's a nice round number and subconsciously gives the user the impression that it is fully charged and ensures the maximum of mobile usage time before the device has to be charged again.
However, this might actually cause more problems than it might solve in the long run for your iPhone. Most mobile devices work with Lithium-Ion batteries and their full capacity is only good for a certain number of charges before the cells become less optimized and don't hold it's full capacity anymore.
In Apple's latest software update, a user can decide to what percentage the battery needs to be charged to and usually set to 90% by default on MagSafe chargers specifically for this reason.
Apple recommends that a maximum charge on an iPhone should be around 90% charged and don't let the battery get below 30-40% before recharging to ensure the maximum lifespan of the Lithium-Ion battery at optimal performance.
Take a look at the video below by the YouTube channel: Apple Explained on Why You Shouldn't Charge An iPhone To 100%...