So from my point of view it's incredibly helpful. I have a child (15 and in Y10) with ADHD and severe dyslexia. Copying things down from the board is not going to happen, her executive function is poor and no amount of detentions or encouragement is really going to fix that.
I can understand some parents whose children are not challenged in the same way may not regard smart phones as essential tools, but for us it has removed a huge amount of stress and worry.
Having the homework app means everything is there for her to use without having to remember. Because there's a parent version, you don't have to constantly badger your child, or worry that they have forgotten stuff or email teachers to check what they should have been doing.
It also means that you can be pleasantly surprised when you get the notification to say homework has been completed without having to cajole or nag. And with the online homework tasks it's often marked immediately and feedback provided on errors. The videos in maths also mean I don't have to spend an hour on YouTube trying to remember stuff I haven't done in 30 years!
In terms of when you give a child a smart phone - basically secondary school is when it becomes essential, but slow introduction and lots of monitoring at the end of Primary does make it less forbidden fruit and means that you are monitoring a child and not a stroppy teenager. We started with an iPod in Y5, then an old iphone which only worked on WiFi in Y6.
Maybe if you live somewhere where they're not travelling much it's also less essential, but for kids in central London travel apps and trackers are useful and help with parental angst when your tiny 11 year old is negotiating tube and train changes on their own in the dark.