To see the effects of your marketing you need to be able the measure the results.
If you have a blog, traditional measurable aspects would typically be number of readers, number of comments, number of new subscribers and/or number of shares. But don't you also want to know who became customers from reading your blog?
Statistics or data is essential for you to ensure that you put your time and money on marketing that yields the results you want.
Why advertise on Twitter if all your existing and future customers are more likely to interact with you on Instagram?
To be able to define if your marketing efforts has been successful, you need to set yourself a target, and it should be a realistic one (and of course based on the statistics you can gather). By setting targets and measuring your results, your marketing will get more targeted (duh) and a lot more interesting.