It's simple, you can not justify any marketing spend if you aren’t tracking the numbers. Even more, if you’re not tracking the one metric that matters above all others: Cost per Acquisition or CPA, then you should pull all of your marketing spend right this second.
Don't get me wrong, there is always a time and a place for other metrics as they are important to any well run campaign but they don't come close to Cost per Acquisition.
Ask yourself: How much do you spend in marketing budget to a get a customer? That is your Cost per Acquisition.
So why is it so important? It is the quintessential metric for determining true return on investment. It doesn't matter how many clicks or how many eyes have viewed your campaign, if it's not generating revenue, or buidling your customer base (which may for some be your digital 'bread and butter'), it's not successful.
At marketing dept. we like to break CPA down to each individual channel i.e. Adwords, Facebook, Instagram etc - this is really helpful in teaching us which marketing channels actually convert and where to focus more of our attention and budget.