You can choose whether to self-host or whether to have someone else handle that for you. As for also hosting your apps: I think the answer to that is the same

You can choose whether to self-host or whether to have someone else handle that for you. As for also hosting your apps: I think the answer to that is the same as for why you might still choose to use an email client even if you're hosting your email server. However, I would expect the majority of people not to self-host, but to have that handled by a party they trust. They would then not be limited by that host also having to support the particular photo-viewing layer they are interested in, but instead are able to choose one independently. (Disclosure: I work for Inrupt, but opinions are my own.)