General competence at your function is fungible — anyone can replace you. Specific niche expertise creates career leverage. Being 'the person who knows X area' makes you the obvious choice when X comes up.
How to choose a niche
Look at where your function intersects with broader business problems. Pick an area genuinely interesting to you. Build expertise over 2-3 years through specific projects, learning, writing.
What this enables
Internal opportunities flow to you specifically. External career options expand (you're 'the expert in X', not just 'a marketer'). Pricing power if you go independent (consultants charge more for niche).
Most workers stay generalist. The few who develop genuine niche expertise consistently outperform in pay, opportunity, and career resilience.