My guess is there is money to be made in fleets. An EV version of a old school fuel card.
Except they don't have any web based access to your account and don't own the bonnet.com (or co.uk etc) domain. The joinbonnet.com website doesn't even give you any way to actually join unless you're on a mobile and download the app. I guess they can add it later on.
But I suspect I'm not alone in downloading it, eventually finding how to log on, might use it for one free charge, and then ignore it forever more.
With contactless payments now mandatory on all new UK rapid chargers as I understand the situation, and cars getting increasing range, the need for an individual to rapid charge will decrease (lots of demand still, but rather than 50k owners charging 3x a week, it will start to become 500k owners charging 1x a month on average). I'd have to use them a lot to make it worth my while and given public charging is a fair bit more than home charging nobody is going to shift. The cheaper introduction tariff was just to draw in customers.
Who knows, they might succeed, but so far I have been spectacularily unimpressed. Try this simple check - open the app, go to settings, go to App information, then try any of the bottom 3 (of 4 links) including Privacy, Terms and 3rd party legal notices - all of them broken links
They've not had many app reviews, but the latest scores are 2/5. 1/5, 1/5, 3/5
Building APIs to 3rd parties is also a challenge and needs their full support, billing and complaints ain't cheap to manage and you have the issues of "who's fault is it anyway" to wade through. Ecotricity couldn't really make a go of it when they had a monopoly and no 3rd parties.
The models I can see being successful are
1; provide the company card as mentioned, but that could just be a company credit card for charging, and/or
2; become the operator for third parties so they don't build their own billing infrastructure, a bit like amazon do in retail.
Neither of those things are on their roadmap that I can see