One of the tenets of a senior engineer is their ability to 'juggle a lot of balls in the air'. They have the ability to switch between different contexts and projects, while still maintaining productivity and understanding of what they are working on.
For example, a senior engineer may be able to pair with another developer, spec out a new project, learn a new tool, etc. all while keeping up on their day to day work. For less experienced engineers, the context switching would kill their productivity and make it too difficult to complete projects.