There are MANY different kinds of devices, and each offer their own unique blend of functionality. The hardest to replace seems to be the phone part - calling emergency services or receiving a text using a laptop might be possible, but takes some setup and just is not quite the same as being able to whip out your phone and already be in a call one or two seconds later. Conversely, neither iOS nor Android are even attempting to replace actual desktop/laptop-like operating systems, and while using a tablet as a keyboard may be possible, nothing beats a true keyboard and mouse setup; although that said, a smart device (either phone or tablet) with a broadband data connection can offer a WiFi hotspot to a laptop, thus significantly enhancing its' capabilities still further. Even some places that offer you WiFi may be super slow, unreliable, and/or do things like change the access code every hour or so, making it more convenient to simply provide your own - plus that's a bit safer too (which would you rather use: a public toilet, or your own at home?).

Having a printout (even if PDF) of a bus or train schedule isn't nearly as convenient as being able to connect with an app to live updates - like "it should have been here an hour ago, what happened, do I just wait further?", plus allows quick deviations like "am I so certain that this (other) route even runs on Thursdays?". Even dumbphones with a data plan could access the Internet, but apps are much more convenient, thus useful for things you do often.

Then again, some of that you could replace with a broadband connection on a laptop. What the smartphone provides is a nice bundle of features that, while each one could be replaced individually, is thought of by most people in the developed world to be often highly useful. Though ymmv - e g. people with addictive tendencies might rather prefer the barrier of having to pull out a larger device such as a laptop or tablet than to have immediate access to everything with a smartphone readily pulled from a pocket; and people who have a desktop at both work and home may want to get by with just a dumbphone for calls and texts.

Funny story: some dentist offices refuse to take patients who do not consent to receive texts and respond with confirmation of the appointment an hour or so before - they apparently were burned so often by people who made appointments but never showed that now everyone has to jump through those additional hoops, and they get enough patients that acquiesce that they can turn away everyone who does not. But if YOUR dentist does not do that, then your need for a device capable of sending and receiving text messages is lesser compared to those who do. It still is worth noting though how common it is to have such devices - most responsible citizens have at the very least a mobile phone, though not all!:-P - and your access to services in public spaces increasingly depends on such, so especially in cities, though they have significant uses even in rural areas as well. And yet if you don't use public transportation, always drive the same roads (or have a dedicated device for GPS, if you need that), and haven't found the need yet for apps, that's fine too? You apparently don't need them while moving about, though they can offer great convenience - enough even to replace a laptop entirely, if you have quick access to desktops.