There are many applications of CTR in marketing, for example a few of the most common use cases include:
CTR on PPC (pay-per-click) ads
CTR in organic search engines (SEO)
Email CTR
On-site CTR (i.e. buttons, etc)
In each of these examples, CTR is an important metric that helps you understand what’s working with your marketing and what needs improvement. For example, if you launch an ad campaign with various iterations of marketing copy, CTR will tell you which one had the highest impact on driving the desired action, assuming all other creative and targeting stay the same. Similarly, in other forms of marketing such as email or SEO, CTR can help you determine your most profitable subject lines, call-to-actions (CTAs), and more.
In terms of what a good CTR is, this will depend largely on the type of marketing you’re doing. For example, display ads typically have a much lower CTR than search ads. That said, as HubSpot highlights “across all industries, the average CTR for a search ad is 1.91%, and 0.35% for a display ad.”
Google provides a useful tip for how to use CTR to improve your marketing:
You can use CTR to gauge which ads, listings, and keywords are successful for you and which need to be improved. The more your keywords, ads, and listings relate to each other and to your business, the more likely a user is to click on your ad or listing after searching on your keyword phrase.