A mentor can help you prepare for a difficult assignment. A sponsor might tell the person choosing its leader that you should be considered, introduce you to a decision-maker, or recommend you when you are not in the room. The difference is active advocacy.
Career sponsorship connects a person's opportunities with someone willing to use influence on their behalf. Advice can be part of the relationship, but advice alone does not provide that access.
Someone puts their judgment on the line
Imagine a senior colleague recommending an overlooked project coordinator to lead a regional initiative. The recommendation can make the coordinator's work visible beyond the people who already know it. It also signals that the sponsor is willing to have their own judgment evaluated through the outcome.
Access to sponsors can be uneven. Informal relationships may repeatedly favor people whose backgrounds and styles are familiar to those already influential. A system that publicly praises individual merit can still distribute opportunities through largely invisible advocacy.
The mechanism can open a door for capable people whom ordinary selection channels miss. It can also become favoritism if advocacy substitutes for evidence. The distinction lies partly in what the sponsor knows about the person's work and whether their intervention broadens a fair consideration process or attempts to bypass it.