As I understand the XFaaS system from the article it functions a bit like a consolidated background worker system that any service can submit their work to and it will (try to) make sure it gets done in whatever SLO is specified. This is usually cheaper because now you can do low-urgency work from service B when there are urgent jobs from service A and vice versa, leading to higher average server usage. For small/medium businesses this is probably not something you'd want, since developing and operating such a system costs more than you save. But if you have a multi-million dollar bill for your background workers alone then it might be worth it.
Maybe we would benefit more if we would have large spikes in resource usage, but we don't.