In this sense, while you may be paying for the traffic on a click by click basis, you're still effectively buying it based on achieving your goals.
Additionally, buying PPC traffic is far more accessible then attempting to get things done on a pay per sign up basis.
For example:
CPC - You bid $1 per click to a sign up page for more information on your product. You get a conversion rate of 10% so after spending $10, you get one sign up and the CPA cost of that lead was $10.
CPA - You bid $5 per lead on the same system and observe the same conversion metrics. Now assuming the clickthrough rate was the same on that ad network, they only made $5 after the same number of impressions shown vs. the CPC model. The effective CPC they made from sending over 10 clicks was $.50 instead of a dollar.
The ad platform will eventually optimize itself to give you a lower share of impressions if you bid the $5 CPA because it can normalize expected revenue/returns regardless of bidding models. This is why with a lot of CPA deals, they require a test period to suss out the economic viability of your bid before they will commit to delivering a large amount of impressions on a CPA model on your behalf.
There are also some niches where there's more ROI in PPC.