I used to be German, and years ago while on a visit to Germany my cousin forced me to install whatsapp. I resisted as I didn't see a need for it. At the time, SMS texts in the USA had been free for ages, while europe was still paying a per message fee.
When the founder of Whatsapp posted on Flyertalk about his original version of the app, I even poopoed it as I thought it was just duplicative technology.
Fastforward to ~2016, I started traveling for work to the UK and India every single month from the US. Literally ALL of my coworkers used whatsapp, so I was happy to have a single place to contact them.
While it isn't my favorite app, there are definitely some good things about Whatsapp:
Pretty much everyone who isn't in China uses it. (hyperbole, I know)
It is end-to-end encrypted by default (which is why I hate telegram and facebook messenger with their non-encrypted defaults).
I like the single, double, blue checkmark scheme. It gives me lots of information at a glance about the status of my message.
I LOVE that I can send a message (say on a plane) where I have zero internet, and it will essentially queue the message and then auto-send it once I have signal. Many other messengers won't even let you type with no connection, or make you manually trigger a re-send.
Things I hate about it:
The interface is ugly.
The DAMNED gifs don't auto-play... ruining the entire purpose of a reaction gif.
Photos/videos are compressed to hell with no easy option (at least on iOS) to send full resolution.
While whatsapp has QR codes, they've not caught on as a way to share contacts in the same way that folks using WeChat use them ubiquitously.
No way to send cash back and forth - what a huge miss. Imagine the potential for sending money overseas etc.
Apps on devices other than phones suck.
Tied to a phone number - let us create usernames like Threema does dammit.
I have Telegram. Nobody uses it, not one single person in my address book has ever sent me a message, it's all WhatsApp.
Except that's a slight lie. I get spam via Telegram. Lots of it and almost on a daily basis, I'll receive spam via Telegram. That never happens on WhatsApp.
Because WhatsApp was ‘first’. WhatsApp was already huge before Facebook bought it. It’s a case of ‘if it ain’t broke don’t fix it’ for most people. Plus it has always been compatible on numerous different devices. My dad started using it when he still had a Blackberry many years ago.