The main point from an investor perspective is that if anything happens to the "ads market" landscape, Google as a whole will be greatly affected. For example, new legislation surrounding targeted advertising or use of personal identifiable information for marketing purpose could have a major impact on Google. And those are not some far fetch and unlikely scenario. Some government are eager to put some rules and barriers to protect user privacy and to limit what can be done with it (specially in Europe), and many consumers agree and also care about "their" data. Legislation is not the only risk, even technological breakthrough could shake the ad industry. For example, smartphone became a big deal in the span of very few years. It took Google and Facebook a couple years to adapt to shifting advertising revenue from web browsing to mobile browsing. If you remember some of the early financial reports from Facebook back in 2012, most of them were disappointing due to the fact that many of their users shifted to mobile browsing, and Facebook had a really hard time find a way to display ads on mobile that would result in a click. Their whole business was impacted and it took them a couple years to start growing their ad business again.
The main point is, if you only make money by selling 1 thing (which is Ads, regardless of what medium they use to show their ads), then if something actually happens to that source of revenue, the effect is major. The more diversified your revenue source is, the less likely it is that a single event will affect your company in a major way. Again, not saying Google doesn't have many products, and obviously, having search ads, youtube ads, mobile ads, (and more) as different mediums help mitigate some of the risk that could affect those individual products, but they are still very much exposed to the risks of a changing market.