It really depends on the family if it's a need or not. For some families it's an absolute necessity, for others it's VERY useful, and for others still it's just an "extra".
In our house, it's a necessity (for several reasons, the least of which is no house line), and kiddo has had one since he was 5 (he just turned 9).
A nice (unintentional) side effect/ benefit of him having one so 'young' (although he had access to mine since he was a toddler and could speed dial myself, my mum, etc.) is that they're NOT cool. They're just boring old normal. You use it to call people. Or text them. Or play music. Or to use as an alarm in the morning. Or, or, or, or. They are as "exciting" as a household landline or cd player or alarm clock or calendar or camera AND the rules got ground in young.
It's also turned out to be DARN useful in ways I hadn't considered when we first got it. He's old enough now (and so much better than I am) that he can do runs on the mountain and no one is standing around at the base (or sitting at the top), and waiting, and waiting, and waiting. Instead; it's NO switching runs without phoning, text (and wait for a reply) before getting on the chairlift. SO USEFUL. Same token, I can pull up the GPS ap on his phone and actually *watch* him go down a run. He was also the first one around a fallen skiier (semi conscious) this winter on a black diamond run and he phoned ski patrol, then phoned me, then waited with the skiier until ski patrol got there. It's not a heavily used run midweek, and it takes about 30-40 minutes to go down it, so the skiier could have been laying there for SOME time before my son even got to him, and 30 minutes more before he rode down to alert ski patrol. Instead, in 1 10 second phonecall, ski patrol was up the run 5 minutes later.
Same token, he and his friends can play army guys in the woods by our house, or he and his cousins (who live on 100 acres) can take the dogs and "go play" for hours (neither of which I would have let him do without a cell until he was older, even though I did the same thing at his age). He can go to the men's room at the baseball stadium, I don't have to shout across 3 soccer fields when he and his friends are running it full length.
So I'm able to give him a LOT more (supervised) freedom, than he would otherwise be allowed to, and I can not only reach him on a second's notice, he can phone for help, or for permission, and failing all of the above; I can GPS the sucker.
In the hospital this spring I could go to the cafeteria without being afraid he'd wake up alone (Don't worry mom, I'll ring you), and his friends could call or video chat him (they were too young for visiting when we were on isolation), and he could call family members (grandparents, dad, etc.) who couldn't be there because they were out of state or at work. (the area code for my husband's work means the room phone wouldn't be able to call it).
At the AMNH kiddo used the museum interactive map to go to the exhibits he wants to. Also in NY or London, he uses the metro or tube maps. He's become VERY good at navigating urban areas (he's also my 'navigator' using the map ap when I'm driving... and SIGH... HE'S the one who figured out how to screen shot his phone and the directions in case we lost signal.
In daily life the people we both call (my parents/ sisters/ brothers/ my husband/his dad/ etc. know from the caller ID who is calling). He uses the voice memo function to record what his homework and assignments are, and then types them into his calendar when he has more time (we're an adhd family). He uses the alarm clock in the morning. The ipod function has all of HIS (aka approved, as we have a rather extensive music library, not all of it he's allowed in) music. He takes photos of zoo scavenger hunts (and so much more). He uses a night sky ap that tells him the names of all the constelations (just point your phone in any direction, and it shows them all, including 'deep sky' if you zoom). The alarm / stopwatch also gets set for electronics time, medication doses, when he wants to hold his breath, the Kindle Free Ap lets him read books while on the bus/ waiting for an appt/ whatever...
The list is near endless. Phones these days are pocket computers, and dirt cheap (his iPhone 3gs cost $50, and is only $10 a month added to our plan, that he pays for himself out of his allowance).
No matter how many other families don't need cell phones, I'm beyond thrilled that ours did, it's so durn USEFUL that kiddo has his own. So beyond thrilled that *I* don't even notice how useful it is unless I stop and think about it, like on a post like this. Because it's become our version of normal.