Failure to clean the iPhone's charging port allows the collection of dirt, thus resulting in the blocking of the connection between the phone and the charger. This may lead to a slow charging process, or even no charging or it may disconnect repeatedly. Overinserting the charger in a dirty port may equally destroy the port, which is costly to have it fixed. Cleaning the port will avoid charging issues and save the hardware of your phone in the long run. You can also read How to Clean Out iPhone Charging Port.