I've often wondered why people would conduct attacks if the attacks don't actually end up doing anything.
The fact that somebody is protected by a DDoS mitigation service should be evident to most of the people capable of conducting an attack.
I used to see a bunch of DDoS against my work servers, it tended to be exactly 90 seconds, plus or minus a bit of clock skew in the generators. The time frames seemed to be about the same regardless of if the attack was successful or not, although when the attacks weren't successful, I am less likely to have noticed.
Usually, but not always, our www servers were targetted, and not the servers actually doing useful work, presumably because it's cooler to attack www servers. I have to say, sometimes pretty dumb DDoS would crush our servers, but it was always fun to look at the sampled tcpdumps and fix the bottlenecks.
There are a lot of DDoS for hire sites out there, so it takes less skill to get it done than checking host records.
Why would you have anything other than web servers directly accessible on the internet?
I work for a chat service, the servers doing the actual work of the service were rarely attacked, we even have a couple other http(s) endpoints that are public, but not www, and those were rarely attacked as well.
Here's one good reason: to test their tooling and botnet. If you've been hitting smaller targets and taking them offline reliably, you might not even max out the bandwidth of your botnet. You have to have something that can handle the traffic to measure how high you can go. Even if not, you might just want to continue testing. It's going to be important if you're selling DDOSes.
Apparently even business tier is not immune, Brian Krebs' security blog was DDoS'ed off Akamai after the then-ongoing mitigation cost ended up being far more than they could agree on, and it took him days to find another provider[0].
[0]:https://krebsonsecurity.com/2016/09/the-democratization-of-c...
Also they specifically refer to volumetric mitigation' as the thing which everyone gets.
The harder to deal with attacks are probably the application aware resource attacks.
A brief DDoS was (not sure if still is) a common method to expose the real IP because the CDN edge servers could often be easily spooked by a brief surge in traffic and start redirecting DNS back to origin. I suspect this is the kind of "protection" they were really offering: your site will still be down, but at least the backend is never revealed to the world.
To make it tangible with made up numbers, imagine we pay $10/megabit per second per month. If our egress (out) is 10Mbps and our ingress (in) is 1Mbps then we'd pay 10 x $10 = $100/month. If our ingress went up to 9Mbps and our egress stayed at 10Mbps then we'd still pay 10 x $10 = $100/month.
Since we're a caching proxy, you'd expect egress (out) to be higher than ingress (in). That is in fact the case. While DDoS attacks push the ingress up, the spread between ingress and egress is so large that even the largest attacks don't push ingress above egress. In fact, even attacks that are many times larger than the largest attacks ever seen would still not increase our bandwidth costs.
This is different from other providers that run separate networks for DDoS mitigation, or only provide DDoS mitigation without providing other caching services. My understanding is that Akamai runs a separate network for DDoS from their CDN, which is why large attacks drive up their costs. We made different architectural decisions, which is why it doesn't for us. And, as our caching services get more popular, it effectively increases the size of the largest theoretical attack we could handle without it driving up our bandwidth costs.
So, yes, there is a theoretical limit of an attack we could not handle today. That, however, is at least an order of magnitude bigger than the largest attacks the Internet has ever seen. And, if such an attack did happen, I think there would be other parts of the Internet that would fall over before we did.
For the record: since we announced Unmetered DDoS mitigation in September 2017 we haven't terminated any customer, free or paying, for an attack they've received.
They do it for "the lulz", for fame, for glory, for breaking in their new botnet, for the brief chance of taking down someone's website... etc.
You sorta have to live with this when you host on the internet.