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Usually they are fitted with crosshairs or a reticle (clear glass disk with an etched pattern) illuminated by an LED. Or it could be that you have a filar micrometer eyepiece.
Possible applications:
1. For use in a small refractor as a finderscope, attached to a larger telescope. Even with a humble 8X finder with illuminated crosshairs on larger telescope working at say 200X, putting an object on the crosshairs would put it in the field of the larger scope. This saves a lot of eyepiece swapping.
Alternatives to finder scopes include Tel-rads, red-dot reflective finders (zero-power) and laser pointers.
2. Back in the days pre-autoguiders, if you were doing long exposure astrophotography, you would have used a guide scope with an illuminated eyepiece fitted with crosshairs or reticle to manually keep it accurately pointing at a guide star. A few diehards still do this.
3. Double star measurements. An illuminated eyepiece fitted with a reticle that has a ruled scale and angular divisions (a bit like a protractor) can be used to visually measure the separation and position angle of double stars. Its not the most accurate way but it works well enough for some.
4. If you were really doing things the old-fashioned way you might have a filar micrometer eyepiece, with internal illumination of the micrometer wires. This gadget was once used to measure things like double stars, as well as the angular widths of planets, moon or sun.
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