What are the purpose of having these logos?
Logos are a point of identification; they're the symbol that customers use to recognize your brand. Ideally, you'll want people to instantly connect the sight of your logo with the memory of what your company does – and, more importantly, how it makes them feel.
Why do companies want you to use their app?
Advertisers. Advertisers prefer apps because they provide more information about a user's habits. This information is more valuable than the one which would have been obtained simply from browsing.