Why do I need ASO, if I can just buy incentive installs?
Firstly, to make a move to the top of the particular search request, your App needs to be in the results of those searches, to do so without ASO is not always possible.
Secondly, if ASO will be effective and well done, your app can be quite high by itself, which will make a move to the top much cheaper.
Thirdly, every search request, that is used to find your application, its one more needed point of growth. Position on search requests depends on many factors, even total number of downloads counts, if it is bigger, then your application will be higher on all search requests that ASO was carried out on.