Amith: Measurability is one of the key advantages of digital medium. It's one of the reasons which has contributed to the adoption and growth of digital marketing worldwide. Tools such as Google Analytics and and techniques (e.g. UTM Tracking) allows an organisation to precisely measure the ROI (Return On Investment) of their digital marketing efforts across various channels including Search, Display, Email & Social Media. That too for various marketing objectives including reach, branding, lead generation, reputation management and online sales.
Here are few examples, which will help you understand this measurability opportunity of digital marketing:
E-commerce organisations are able to track their Cost Per Acquisition (CPA) through various digital marketing channels across different categories of products.
A services firm (e.g. IT services) is able to directly measure their Cost Per Lead (CPL) for different digital marketing channels
Consumer Goods (e.g. FMCG) companies are able to measure their brand reach and perception through various digital media across different brand categories
Hope this helps? In case you've a specific question related to ROI of digital marketing, feel free to ask me.