Why does Nexus Mods use Advertising?

To understand why Nexus Mods needs advertising revenue we first need to explain where we are in the grand scheme of things, what we are, and how that affects things.

Nexus Mods is an extremely large and popular resource on the internet. Currently we're ranked in the top 600 most visited websites globally, and in some countries (like the UK and Germany) we're in the top 500. Let that sink in for a second. Of all the billions of websites on the internet, Nexus Mods is in the top 600 most visited sites out there.
On top of all this traffic, we're heavily focused on file sharing (the downloading of mods) and database intensive operations (highly customisable searches for said mods). Both of these site attributes are particularly expensive compared to a website that simply offers static content, with no heavy database operations or file download bandwidth to worry about. That is to say, if Nexus Mods was a wiki that simply served text and images with no file database with the same amount of traffic, our expenses would be 10-20 times less than what they are now. So a site like Nexus Mods is orders of magnitudes more expensive than more static websites out there.
So not only are we popular, but we're dealing with an extremely high volume of traffic on an extremely resource intensive style of site.
If you're wondering what Nexus Mods needs money for, we've dedicated an entire page of the site to an infographic that puts it all in to easy to read data points. You can see that page here.
But why advertising? Simply put, advertising still remains the number one way of generating income on the internet. It's also consistent. In that, typically, the more popular the site, the more page views you get, the more advertising money you receive. As a result, the hope is that the expenses relating to the site will scale with the advertising income you're generating from the site.
Case-in-point; when major game launches happen, e.g. Skyrim SE, we'll receive 25%-40% more traffic to the site over the period of a couple of months. This likely means we'll push more bandwidth, which means we'll have higher costs. However, this should be offset by the fact we've been receiving more traffic, and as a result we should be generating more income as a result. Sometimes it doesn't work out that way, for example, the advertising payout per user may be lower that month for some reason resulting in not making as much as you might have done in previous months, but typically, it should follow that more traffic results in more expenses but also equals more ad revenue.
People will rightly note that we also have a Premium Membership (and Supporter) system. This is absolutely correct, we do, and this service is extremely important, financially, to Nexus Mods. Having said that, Premium Membership alone would not cover the running costs of Nexus Mods and it relies a lot more on the generosity of an extremely small percentage of our users. Not only would it be unfair to rely on so few users for all the running costs of the site, but advertising also ensures that anyone and everyone, irrespective of their financial situation, can contribute to the running of the sites that they're using.
Lets not forget, that while many people bemoan the fact they have to see adverts at all and will actively find ways to remove them without helping in any way financially with the sites (e.g. by using an adblocker instead of paying $2 for a lifetime Supporter membership), many also appreciate the role they play in funding the sites, by not using an adblocker, when they can't contribute directly themselves.
It's our responsibility to ensure those adverts are relevant and secure while trying not to distract from the actual content of the site. When we fail to do that, we indirectly encourage people to explore ways to avoid seeing the ads, which is not in anyone's best interests. If everyone used an adblocker, Nexus Mods would cease to exist, after all. Currently, 45% of Nexus Mods visitors use an adblocker of some description.