When we use a candle, we’re using fire. Why do we use fire??
When we’re using a mirror, what does it do? What is the point of a mirror in magick?
Hiya OP! Fire is used as a means of converting energy types through passing into and out of the flame, as well as acting as a focal point for the practitioner’s consciousness. It’s also the element of awareness, and by fixating ours on a corresponding power the natural energies of both are strengthened for magical use.
A mirror’s function will vary depending on the person and context in which it is used, and can be anything from a scrying device, to an anchor for portals, or an interface for spirit communication. The imagination’s the limit! 
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They are magical gateways for the energies to pass through and manifest in this reality!