This is same as of the other similar files that you find on the internet. It is legal and it is completely safe (as long as you download it from the official website of the company). We do provide you the apk file download link directly from the official WhatsApp website.
It is true that you can find the application in the app stores in any and all of your devices. Well here is the case. When it comes to these ones, developers try to fix the problems and bugs as well as add features to them as much as possible. They add at least two or three modifications every week in order to assure that their product stands in the top of the list. But it takes time to the official one to be updated with all of them and to be uploaded to the App Store. In this case, even if we think that it is uploaded it takes time to reach all of the customers.
The updates are distributed in a queue so that the time that the countries get they differ from one another. It at least takes two weeks to completely reach each and every single user in the world.
As a solution to this problem the developers always upload Beta versions of the app on their website. This helps them in two ways; first, it allows them to test their updates and get the feedbacks of the customers who downloaded the APK files. Second, it helps them to reach as many customers as possible way before the update hits the play store. Hence, the rush is quite minimized.
The latest version that is available in this format is WhatsApp 2.12.360 which is still in the test mode and hence available as a beta file.