There are lots of beliefs and variations on beliefs about mirrors in a magical context.
For a long time, there's been a theory that your reflection is literally your soul or the expression of your soul. As such, mirrors have been tied to a lot of spirit myths. Traditions like covering mirrors after death and not allowing babies to look into mirrors have been around for a very long time. (This sometimes extends to reflections in general as seen in the story of Narcissus and some of the African myths,plus it can also apply to photographs and images of people in general). In some cultures, mirrors (or reflections) can provide another vessel for souls. I'm not sure how much stock I'd put in these stories but certainly I have seen/heard of people using mirrors to confuse, bind or attract spirits. The method of doing so would depend on your personal practice. (Also, that kind of stuff isn't my expertise so I'm afraid I only have surface knowledge of those rituals). Theoretically, mirrors could also be used to curse but again I'm afraid I don't know much about that.
If you wish, you can also use mirrors for scrying. There are other methods of scrying and the mirror/reflective surface isn't strictly necessary but can be a useful aid for some people.
I personally tend to use mirrors in rituals focused at myself or my direct environment. However, depending on the context, the symbolism of the mirror can differ. If you're a "it's not the tool, it's the focus the tool brings" type of person, I'd suggest using a mirror whenever you feel it's important or relevant to the ritual/spell/whatever. If you're a "the tool makes a difference" type of person, I'd suggest reading up on superstitions more specific to you path and go from there.