It may have changed since then, but last time I looked at robosats the market to transact BTC to JPY or JPY to BTC was not quite there, same for other platforms like Bisq. Furthermore to pay on Internet in many cases it was a hassle without a credit card or without installing a non open-source app to do a bank transfer. So what remained as a choice was cash. And with Vexl you can use it to create your peer to peer network to transact with cash and sell or get bitcoins.

With Vexl also you can ask the identity of someone so you are pretty much free to choose any means of transaction. It could be BTC to gold, BTC to JPY in a cafe or meetup. Vexl favors these kind of physical ways to do exchanges like this and with Vexl this is at your discretion. With robosats I think this is more Internet focused.