Why might you use the “Search Network campaigns with Display opt-in” campaign type?
Your video ads can run on the Search Network
You can use one budget to advertise on the Search Network and Display Network
Your ads only show on the first page of search results
You can pick the exact websites where you want your ad to show
The “Search Network with Display Select” campaign type helps you reach people as they use Google search or visit sites across the web. You manage your “Search Network with Display Select” campaigns the same way that you’d manage a “Search Network only” campaign: set a budget, choose relevant keywords, create ads, and set bids.
Read more here: https://support.google.com/adwords/answer/3437721