Do I even need a cover letter for an internal role?
If the internal process asks for one, treat it as seriously as an external application. It's your chance to make the case for readiness rather than assume it's obvious.
How is an internal cover letter different?
You can reference specific results and people who can vouch for them, and show institutional knowledge as a head start. The trade-off: vague claims are easy to check, so specifics matter more.
Should I mention how long I've been with the company?
Briefly, as context — not as the argument. Readiness is what earns the role; tenure alone is a weak case for a promotion.
What if I'm applying to transfer to a team I've never worked on?
Treat it a little more like an external move: lead with transferable results, and use your knowledge of the wider organization as the bridge into the new team.