Why cellphone have internal and external memory?

Internal is normally dedicated to the phone and it's OS. For such use as adding more apps, texts, contacts. External is more pointed at using it for your own personal media. although on smartphones you can point apps to be loaded from the external memory. other factors are if you can take the memory out of the phone. Internal meaing you can't and External meaning you can. This is normal in the format of a memory card. (SD, Micro SD etc)