It's an issue of convenience. Say I am using my iPad in my living room and my phone is in my bedroom. With WhatsApp, I always had to make sure that my phone was right next to me so I didn't miss out on any incoming messages. With telegram and iMessage, I get them regardless of which device I am on. I can effortlessly receive and send texts from my iPad or Macs. Even my Apple Watch.
And it's easier to type longer message on a computer compared to a phone.
For me I never really have that problem because I habitually have my phone with me or around reach most of the time.
Using Slack as an example, if I'm away from my desktop, I usually get pings to my phone. In other words, my iPhone is always the end-all be-all receiver of all messaging.
In my opinion, it is one of the essential things most people carry with them on a daily basis. Arguably, I might be one of the few on this forum that does this.