[Jensson]

Senior engineers can solve technical problems juniors can't. Senior engineers can provide technical leadership and ensure juniors don't get lost. Seniors are great at product management so keeps your product on track instead of wasting time building features you don't need.

At least one of the above or similar things. The important part here is to not think that you need anything in particular to be a senior engineer, there are lots of ways to grow the value you bring to a team and it is much better to have a diversity of senior engineer types than to focus on any one kind.