Media planning is an art and a difficult one at that. Placing ads on specific websites, based on demographics and target market, as well as audience reach can be very helpful. Banner advertising has been around a long time and certainly has its purpose, as well as a place in your digital marketing plan. However, it should not take the place of traditional search. Media placement is a compliment to keyword search, or vice versa. They should be considered in partnership with one another and if done right together, can be incredibly successful.
One of the easiest ways to create the dual-approach digital marketing strategy is via Google AdWords. This platform is most famous for (and very effective at) keyword advertising, however because of their vast Display Network (spanning many categories and audiences) it is also very efficient for site targeting and media positioning. Given the ease of entry into advertising on the network, minimal ad spend and control over budget, AdWords allows businesses of all sizes and revenue to test this medium. Additionally, the "per click" cost model keeps expenses tied to actual user engagement (clicks to the site versus just impressions of the ad).
Of course you will need to analyze your marketing budget to determine what the optimal advertising mix is. Given your established goals and objectives, there may be reason to support a higher ratio of one over the other. In many instances, being able to simultaneously create branding and awareness (through banner advertising) while also engaging with users when they need or are researching your product/service (keyword search) is the winning combination.
On the next advertising campaign you launch, make sure you have the most immediate and engaged users covered in your digital marketing plan. Keyword search should never be underestimated, when it is a product or service users are searching for. This can be validated through Google's keyword search tool and the historic volume on a term. Be where the audience is seeking you out, more so then where you think those users might be.