Depending on who you ask, it's one or more of: 1) So that the "free articles this month" counter doesn't decrement for stories you don't actually read. 2) So

Depending on who you ask, it's one or more of: 1) So that the "free articles this month" counter doesn't decrement for stories you don't actually read. 2) So that they get metrics on which articles people actually read vs. loading in a tab and forgetting about. 3) Forcing an interaction manipulates your psychology so that you pay more attention to the page, increasing engagement.