When interviewing for a senior network engineer's role, the interviewer assumes that you started in an entry-level position within the network organization and advanced through the organization based on your experience and acquisition of new skills. They will ask this question to allow you to describe what you learned during this process, even though they may already be aware of this at a high level, having read your resume. This is the opportunity to highlight your skills and experience, which will directly benefit their organization. Make sure to emphasize your team leadership skills, which are important to this role.