It's aimed at covering the server's costs but also paying us for the time spent on the project (development, maintenance, promotion, ...).As said by @sebbbl, it's the cut we have set for now.

It may (and surely will) evolve with time and the use of the service. We may also put a part of this fee directly on the @steemigram account to increase Steem Power and the value of the upvotes that will be given to the users (Curation is not up yet, but it's only a matter of a day or two).

In the event that the service is widely used, this percentage may decrease, but for now it's really just there to try and cover the fixed costs of such a project.