Because all (or almost all) people are on mobile now. It doesn't matter if they're pre-teens, teens, adults, or baby boomers. They use mobile devices. And not just mobile devices but smartphones.
They use it to communicate, to get information, to do business, to market themselves, to do presentations, etc.
And it almost doesn't matter what industry your business is. Most certainly than not, you will need a mobile application or a mobile presence.
If you're in the medical industry,
you will need your potential users to be able to easily browse and discover you while they are on the go (a.ka. on mobile). Or you will want to really supercharge your medical business and be able to let your users book appointments with you based on your available schedules, or notify your users when you have an announcement to make. These are just a few examples of how mobile presence can put you above the rest in the medical industry.
If you're in the fashion retail industry,
you will need your users to get notified of your promos and events. You want your users to be able to see themselves wearing your latest products just by using their phones. You want your users to be able to pay using their smartphones after they have browsed your catalogue in your mobile application.
If you're a hotel owner,
you will want your users to be able to browse how beautiful and top of the line your amenities are. You will want to guide them in your hotel in such a way that it will be very tempting an experience for them. You can do this with your mobile application's virtual reality. And after they're so much engaged in their experience in your mobile app, they'll book right there and then, through your app.
And many other industries (actually almost all if not all) will benefit greatly from having a mobile application. All it will take is the creativity to leverage your strengths and put it in the hands of your market.
So what are you waiting for?
Give your users mobility, better user experience, and innovative ways of connecting with your products and services through your mobile application. And in return they will give back their loyalty, patronage and amazing word-of-mouth for your business.