I wouldn't get a new iPhone/Apple Watch for a one time Tokyo visit, especially if using a XX hour visitor subway pass, but it has made it easier to take care of the mundane cash & IC balance check.

90% of my Apple Watch's reason for purchase was Mobile Suica. I refer to mine as my $$$ Suica card. It is incredibly convenient since I tend to go towards the countryside and/or leave the usual international tourist route. It is also really useful on longer train rides to set a timer ~ 10 min before I need to get off. There's a few other things like health tracker, iPhone camera trigger, ability to show a few time zones, and quick read of iMessage which is helpful in everyday life. Apple Watch 3 would do the trick and with version 4 rumored for Sept 2018 announcement, version 3 will probably be on discount soon.

Apply Pay also works with a few other Transit providers outside Japan
https://support.apple.com/en-us/HT207958