> Why should I understand Tumblr or any other product? I and others as well were happy with just reading the ramblings of other Internet users.
I mean, no one's forcing you to. That's my point though!
Casual web visitors to the public-facing (no account needed) portion of the product simply aren't exposed to the majority of the product, and have a sub-par experience as a result. This reduces the likelihood of the product being successful. Some products try to solve this with a hard login wall, but that's problematic for both casual viewers as well as a portion of the content creators. There's no good solution.
In the example of Tumblr, even if you just want to consume content without posting anything, the reading experience is much better in the logged-in dashboard (newsfeed / activity stream) vs separately visiting random blogs that all have different themes.
> As for the other side, publishing your texts or photos on the Internet for others to view, isn't enough?
If you want to just publish photos, you can use a product that specializes in this, rather than choosing a combination publishing site / social network where the community interaction is inherently a huge part of the product.