Imagine an scenario where you have someone's phone number that you need to contact just once (book Airbnb, BlaBlaCar) and hence you don't want to save it as a contact to avoid Telegram or other installed apps to import it. As you don't know if they use Telegram the only way to check it is by adding them to your phonebook.
On WhatsApp you can directly open a conversation using a link: wa.me/[number-in-international-format]
The ability to create URLs with phone numbers to open chats without having to save them to Contacts would help in this case.
Since usernames cannot start with a number there would be no collision.