Keywords is the main thing in ASO, and source of traffic for any app, just like it is for SEO on websites. You need to think about what keywords you want your app to rank on, even before it's released to the app store.
Buying keyword installs can benefit your app in many ways. It will surely give you more organic traffic, which will lead to you earning more money. But it will also as you get higher ranks, put you in a better position to fight against your competitors.
Earn more money
With the right strategy and keywords in the right positions, you will surely get more money in the bank, and the more money you earn, the more keywords you can promote, which leads to an never ending circle of success.
Put you in an overall better position
Even when you've got some good ranks, and fall a bit of if you're stopping the installs, your app has already got a lot of benefit from the installs, because the more total installs you have, the easier it is for your app to get better ranks again, or by just organic traffic.
Making your app competitive
There's millions of apps on the market, competing for success on different parameters. The big players are always going for the most creative way of promoting apps, which could be buying installs, and therefore you should do it too, in order to be competitive.