Interestingly, the web-based activation page was not originally designed for the general public. It began as a back-end tool intended for developers and press. Journalists reviewing games and Kickstarter backers needing to redeem bulk keys often needed a way to attach licenses to their accounts without immediately downloading gigabytes of data. The browser interface provided a frictionless way to "bank" a game for later.
This method is often used by official retailers like Fanatical to offer "one-click" activation buttons after a purchase. Key Things to Know How To Redeem Steam Keys - Humble Support steam activate key browser