Eyepieces magnify the image formed by the telescope's objective lens or mirror.
All telescopes need eyepieces, and you can use the same kind of eyepiece for both reflectors and refractors. However, there are many different kinds of eyepieces, and each have particular features. In addition, even for the same kinds of eyepieces, there are tradeoffs to be made for weight, cost, quality, and so on. The magnification given by an eyepiece with a telescope is simply the focal length of the telescope divided by the focal length of the eyepiece. You can get any magnification by choosing the right focal length eyepiece.