Say you're out eating with mates and you want to split the bill, no one really carries cash anymore here so that's not an option (and also annoying to split). Writing a cheque is obviously ridiculous and a bank transfer requires a lot of needless information. A "swish" as we call it takes about 10 seconds, 5 if your friend is already in your phone's contacts.
It's just an ease of access / quality of life thing. Your question is more or less equivalent to: "What's the purpose of QR-codes? Just enter the URL!" - it's easy, fast, and frankly nice. A really really simple and great way to send small amounts of money between people.
