I used to work in a mobile phone shop and we had these to transfer data between old and new phones. With regards to the SIM slot, SIM cards can store around 100

I used to work in a mobile phone shop and we had these to transfer data between old and new phones. With regards to the SIM slot, SIM cards can store around 100 phone numbers and many people that swap phones regularly do keep their contacts on a SIM card. SIM cards can wear out over time so this was useful for swapping phone numbers from an old SIM to a new one or a nanoSIM etc.