Why erase data on iPhone before selling when not including the sim card?

There are a million posts on how to erase personal data, but is this necessary if you're not including the sim anyway? Is personal data stored elsewhere? ThanksWhy erase data on iPhone before selling when not including the sim card?
Sim cards exist only to store your phone number and data like contacts and such but sims do not store information unless you actually tell it to store info. By default all info is stored on the phone's internal memory. So when you sell it, all your info needs to be wiped clear because it was never on the sim card. To keep your data you need to, before wiping it, xfer all your contacts to the sim card so they appear on the new phone when you insert it.