Well, with a cell phone from Japan you can text Japanese people and you can pay maybe lots of money to do international calling unless you use Skype or Line etc.
Ill just tell you what i plan on doing.
I plan on going to Japan with my laptop and american cell phone.
With my american cell, i can still use wifi to contact my family using skype. So itll be more of a mini laptop.
As for my laptop, itll be also for skyping but also for school work.
As for when it will be need, i think you should bring your cell phone from home at least if it can use wi fi. So in case you are kind of lost(which is rare im assuming), we have each others skype usernames and we can contact each other. For example, my phone and most others i think will pick up any wi fi connections nearby.