What happens is that when a developer submits an app to the App Store via iTunes Connect you have to select the countries in which you want to be or not be available. 

By intuition you want your app to be available everywhere. But this is not always the case. For example, there is no point in making an app of a local newspaper available for all countries, or an e-commerce company delivering only in a specific geography.

• expatriates
• people who can speak your language
• journalists 
• bloggers who want to cover your app
• people who travel to your country often (business or pleasure)

For example the app GET TAXI (a great app to order and pay cabs from your iPhone) is available in the Israeli and the US app store but not the French store. Which is a mistake because there are more tourists in Israel from France than any other country.

By restricting your app to a set of countries, you only limit the chances of your app getting downloaded many more times. As a result some users create often a second iTunes account for the relevant local store. Which is a painful experience when you need to update apps here and there... 

Sometimes this restriction from developers is motivated by good reasons

• The developer does not want to frustrate the users with a limited experience
• The developer thinks users will get angry because the app is not available in other local languages
• The developer sometimes simply ignores how big the market is outside the USA (or his own country) and prefers not to bother
• The developer prefers to control the load of usage on his app or prefers to have local test in a country before a global release (e.g., this is what VIBER did at first)

But really there is nothing at all to lose by making your app available in all countries. Do not underestimate the number of users able to speak your language outside of your country...  

• Make sure to include the initials or country code in the title of your app that if the app is only available for one country, e.g., GET TAXI (IL)
• Make sure to start your description with a disclosure. E.g.,: "This app is mostly for use in the COUNTRY X but we thought it was right to make it available to all users in case you would travel to this country, or simply live abroad but are native to this country"
• If your app is not translated but answers a universal need, just say that a localized version will arrive soon.
• It costs nothing to add a note that your app will soon be translated (you will translate if you see enough local traction)
• Even if you do not translate the app, start with a few international greetings. "Hello, Bonjour, Buongiorno.." to give a sense that you care..

Again this costs you nothing. A few extra clicks during the submission process and the payout can be big.