Why is it important to measure return on ad spend?

This post should be 1-3 paragraphs and should include at least 1 question that you still are curious about or don't feel completely clear on after writing your post.

For example, you might focus on the question "why has marketing been difficult to measure?". Write 1-3 paragraphs discussing your thoughts on this topic and then you might end with something like "a question I still have is whether all the new technologies and digital analytics actually make us any better at measuring marketing, or if we still have all the same problems".