A huge proportion of consumers will use their portable devices to access content, more so than computer and laptops. How many people do you see walking along the street with their mobile phone or tablet and how many conversations do you overhear about them looking to purchase something? If you listened, a lot.
Why do you need a presence on IOS or Android?
I think the question is, why haven't you already got one? If you have gone as far to get yourself a website design and probably paid someone to do it then it is worth considering the latter when it comes to moving into app design and responsive web design. You already know a huge amount of the population use mobile phones to access a huge proportion of the internet, so they could be accessing your website and buying products on the go. In 2017 the IOS/Android development market is crucial to getting you more customers.
If you are thinking about going into IOS/Android development, there are many options available to you. You could use a free builder to help you build an app from presents (and probably lots of adds too) or you could get a web deign business to build one for you.
What is the difference between an App and Responsive Web Design?
An app is a standalone version of your website which integrates with it. Whereas if your website has a responsive web design it will automatically change the way it is displayed to suit the format it is being displayed on when using a web browser.
IOS/Android development - Which one first?
According to research IOS users are often younger with higher educations and the IOS market consistently earns more revenue but the Android market has a larger platform share and thus more available than IOS. There are a number of companies that choose to release their IOS app first and the branch into Android, but it is personal preference. If you are thinking about having an app built, then consider choosing both and hedge your bets in the market.