A mobile app runs on the mobile phone platform creating a user shortcut and an interface that has been optimized for the mobile device that is being used. It then can access the same content that you would access using a website. There are benefits and drawbacks but the main thing a client should think about is trying to make a website at least hybrid mobile friendly.
If you are planning on mobile app development you should talk with us about the benefits and drawbacks to mobile apps. In most cases a mobile app can provide a great benefit for the business but in some cases a responsive website will be enough to market your product and services.
Keep in mind there are many applications and situations where mobile just does not play a role because of the size of the devices and the lack of user interface. However, this is getting better with mobile bluetooth keyboards and other devices that help you control your mobile device.
When was the last time you typed a 100 page research paper using your mobile phone. Besides losing your eyesight your thumbs will probably not endure the challenge. External keyboards are available but that is no longer a truly mobile device and with a keyboard most people would prefer something with a bigger screen which is more like a laptop. How many times did you access a website that required interaction that a mobile device was just not capable of performing or that you could do a lot faster using a PC. Mobile can only get so big before its no longer a mobile device and turns into a mobile PC.
Could the future change everything to mobile and require website developers to change everything to a mobile screen world? Maybe but future mobile will most likely reduce the size of a mobile device to a small watch. This would allow users to view their content on any size screen they want such as their car windshield, a virtual wall in their home, the streets your walking on, a fold-able mobile flat screen TV or any viewing device that is in their presence. Viewing content will be the focus of mobile devices while user input methods will need much improvement. In the future any surface can become a monitor and everything will be driven by voice command. Advanced voice command will drastically improve mobile interaction. When we get to that point then yes every website will need to work mobile, be viewed on any device it is linked to and needs a quick accessible app to drive it.
We are not far from this type of technology and already see some of these devices today. A good example is solar roads with streaming digital screens integrated into sidewalks. Today it would be a big mistake to not think about mobile devices when doing website design.