Return on investment measures financial gain against cost: net profit divided by total cost, times 100. A campaign that cost 1,000 and generated 4,000 in profit has a 300% ROI. It is the most general measure of whether an activity paid off, and it forces marketing to be judged in the same terms as any other business investment.
ROI matters because it connects effort to money. Reach, engagement and follows are means, not ends; ROI (and its cousins ROAS, return on ad spend, and social ROI) is what tells leadership whether the channel deserves more budget or less.
To calculate it credibly, count all costs — media, tools, and the time of the people involved — and attribute revenue with tracked links, promo codes or a proper attribution model. On social, be honest about the long, assisted path from first view to purchase rather than crediting only last-click sales.