The "circle" inserts with the different sizes are for different size circular targets/bullseyes at different ranges.
These globe sights with circle inserts are used with an aperture rear sight, so you have double apertures. These are used with round
bullseyes, as has been noted. The perfect set-up is an even reduction in "light' around the 3 circles, rear, front and bullseye, so they
appear to be perfectly stepped. I was able to shoot a 7/8" group at 100 meters with this setup in an 1874(ish) rifle with 10 shots. As
noted, this is a very accurate setup for target shooting. The rear aperture, if used with a blade or bead makes for a good hunting sight,
but the rear aperture needs to be large enough to admit enough light to see the front sight, in shaded bush, for example.
When I was shooting modern guns prone competition, I found double apertures, rear and front, allowed me to shoot pretty much the same
sized groups at 200 yards (1970's) as I was able to shoot at 200 yds, when the gun was scoped.