It is not necessary to have a mobile app for your business. It can just facilitates your business. This doesn't mean it's necessary. Yes, mobile friendly website is necessary for a maximum reach. Though it is advisable, if you can have a mobile app then better in terms of customer interaction.
This is the era of smartphones. Maximum time of your customer is spent on a smartphone. So, why you don't want to be there! It is just like living in the moment. Uses of mobile apps has risen exponentially. Why not your business exploit this opportunity! A business goal can be achieved even without a website. But, you are having a website. It also is not necessary for a business. It's an asset of your business. Which multiply probability of getting seen by your customers.
Geo-location, bio-metrics, cameras, sensors, augmented reality, 3D gaming… these are potential game-changers that already exist natively in mobile apps — features you won't find on a traditional website. Not only that, but new functionality that we haven't really thought of is going to be available to apps much sooner than they are for a website, if ever.
You must have ever thought, why some companies are app only based reach! Though I am not in favor of this approach. All doors should always remain opened for all customers. Mobile apps give a life time valued customer base instead of one time shoppers. Due to this ability of apps, few e-commerce companies went app only.
By analyzing the nature and feasibility of your business, you can reach to a decision. Good Luck.
Christina Siegl, Mobile App Developer