What reason is AWS premium support mandatory? I ask because I'm currently building out SaaS offering on AWS and haven't yet hit any issues requiring support. Can I expect to start seeing issues as traffic scales up to a certain level?
It's not. It's utterly useless. After an entire year, where the support has never been of any help, my last action for the year 2016 was to call a meeting with everyone, subject line "we should cancel our support subscription with AWS".
It's clear that we (especially me) are way more qualified in all AWS offerings, from basics to special quirks, than they are. And they can't do anything that we can't do ourselves.
I think the support is useful when you first start out, they can answer a lot of general questions. You should subscribe for support the first year and see how it goes.
Note that some AWS managed services (e.g. RDS) can only be debugged by the support so you might be forced into support if you use these services. We don't.
Still, it really depends on what you're doing at which scale with AWS. I also found business support quite good, if you don't need such dedicated resources.
Regarding seeing issues, it's simply a matter of probabilities: The more you run in AWS the more likely it is to get problems sooner or later with one or the other service.
My advice is to start with business support and see based on the experiences with it if you have additional requirements business support doesn't meet. If yes, you might then better know if premium support might offer enough value for its money.