Sales & Marketing Alignment is something that every business has to do, it's just a matter of how well you do it. In its purest form, Marketing & Sales Alignment is the agreement between your Marketing and Sales departments about the process a lead goes through to become a customer, how that is tagged and tracked, who's responsibility it is to move them through that process and what constitutes a quality lead. Think of it as an extension of your buyer personas, from the perspective of how they should be interacted with by your business.
We consider it essential because even a business that has never openly discussed it has some sort of give and take between marketing and sales - there is no way for either to function in a vacuum. If you want their relationship to be effective, efficient and optimised however, you'll need a formal alignment to ensure that everyone is on the same page.
Case in point, how does a marketing team know which leads are valuable and likely to close without discussing with the sales team. How does a Sales person know that it is there job to follow up on a lead that has passed through the marketing if that hasn't been discussed with the marketing team? Marketing & Sales Alignment gives businesses the opportunity to focus their staff and ensure everyone is moving in the same direction.