Why do companies buy ads when they are the top result in search?

Biggest reason is to keep direct competitors from leveraging your brand equity to steal traffic and sign ups. 

You would be surprised how little thought actually goes into ad buying past "I have a budget I must spend", or "I must be buying Facebook or I will look stupid". Dane Madsen Dane@DaneMadsen.com 206.900.5852 Mobile Sent from my mobile device. Forgive typographical and grammatical errors.

Primarily to prevent conquesting by their competitors. Ford for example is the top listing for "Ford cars"  but they do not want Chevrolet to buy the ads above their organic listings. 

There are several reasons this is a great idea...

1) There are now typically 4 ads at the top of the page followed by 10 organic results followed by 3 more ads.  So there are 17 listings in total.  If you rank #1 organically, at best you are #5 on the page (many don't realize the first 4 are ads, many others don't care).  So having an ad gets you farther up the paid+organic SERPs... closer to the top of page 1.

2) When users see your brand showing up over and over for their related searches and multiple times for a single search, it builds brand and credibility/trust.

3) When you buy an ad PLUS have 1 or 2 organic listings on page 1, you have 11.76% or 17.65% of the real estate on page 1 of the SERPs, respectively.  If you only have ONLY 1 or 2 organic listings with no ad, you have 5.88% or 11.76% of the page 1 real estate. 

An ad will double your real estate if you have only 1 organic listing on page 1.  It will increase it by 50% if you have 2 organic listings. By having an ad in addition to the organic listing(s), you also eliminate one of your competitor's ads from showing on page 1.

This has always been a wise strategy.  This typically yields more traffic than if you only have 1 ad or 1 organic listing. And you'll typically end up paying less with both an ad and an organic listing than if you were running only an ad because some visitors will end up clicking on the organic link instead of the ad.

Has it occurred, perhaps you won't see squarespace as top result otherwise? Sent from my iPhone