You use this structure when you're describing something with an adjective, but you think your description needs more explanation. In the example sentence at top, you say that modern life is "confusing". But that could mean a lot of different things. Why is it confusing? Which part of modern life is confusing? Using "in that..." makes it clear what this means.
What follows "in that" is an independent clause, which basically means a whole sentence. This is a sentence:
It was announced 6 hours ago.
And here's that sentence used with "in that":
It's already old news, in that it was announced 6 hours ago.
This sentence (taken from a blog post by the musician Moby) is supposed to be a bit of a joke about how fast news spreads these days.
"In that" is used more in written English than in spoken conversation.