Placements and keywords together decide where your ads should feature in the display network of Google. The ads can be created using keywords or a combination of keywords and placements. In case of ad groups on display network you have many choices to show your ads, however, using combination of keywords and placements is the best option. If you use keywords in the first instance, you can add placements later. Moreover, after adding placements, you can also decide to stop using keywords if you want. Thus, the use of keywords and placements give you enough options to show ads rather than limiting yourself in one choice.
If you want to choose specific places where you want your ad to appear, using placements is better for your ad group. But if you want your add to be placed close to the relevant content, keywords is the right choice. However, the best thing is to use keywords and placements together for your ad group because in that case keywords immediately work on finding pages with relevant content, which is said to be contextual targeting.
In case of multiple webpages of placements, your ads are featured in those pages where your keyword matches the content. Yet, it is important to know that when using placements and keywords together, the combination doesn’t affect the appearance of your ads on the Search Network. Overall, Broad Reach and Specific Reach are two main ways of targeting your ads with placements and keywords.