Good question! Phones are usually the primary computing method

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Good question!

Phones are usually the primary computing method in India. Homes are small, so full on computers are less desirable.
Many services are available using the phone as well, though I cannot remember if they use WeChat or something like that. Or if they use apps.
Computers are expensive devices. Having some form of laptop is possible in many countries. An ergonomic desktop solution for yourself is a pretty Western idea.
Phones, on the other hand, are cheap. This is because of Android in two ways: Android is practically free so phones can be gotten for low low prices, and it's also far more free in the ability that manufacturers can make their phones with low end hardware. Think of those Moto E phones, or all those brands you don't often -if at all- in Western stores.
On top of that, to make use of all these services and to have it be a chance of replacing a laptop or a desktop, it has to be open and capable enough. iOS does not have a file manager, for example. The problem with that is that it becomes more difficult to use "advanced" features such as file upload, like using the Safari app to upload a pdf file to print. That is possible on Android and Windows, but (until recently?) not on iOS.
There's also a ton of advertising and wanting to make it to the west, to work at Google or Microsoft.

So it is a combination of price and usability. Advertising and the fact that there are so many people there that it's unavoidable to see them.