As a senior developer, one must be able to see the bigger picture. Reading books that you’ve mentioned is just what helps in that. Good books cover the amount of concepts that allow a developer to see where they’re moving with what.

Senior developers, hence, guide their juniors. They tell them where and how the system will work and how it should be designed. Junior developers implement the details over the ideas framed by the seniors.

Considering StackOverflow, in most cases, it relates to micro issues like “how to change the color of this button when the user types in this specific string in that textbox?”. It relates to things you do to accomplish a software as a complete system.

Any decent literate individual will be able to follow instructions from StackOverflow. Just because they made things compile doesn’t mean they’re engineers. Its similar to how a person can cook, get ingredients, follow instructions.

Moreover to this, a senior developer will be able to create synergy and induce enthusiasm in his/her team for the project they’re working on.