Not sure what the use is- suppose it is meant to increase the light gathering power but would be nice to see what someone who has actually used them has to say
Btw cheaper alternatives are available on AliExpress
AU $82.79 37％ Off | Binocular Stargazing Mirror Outdoor Camping Telescope Large Eyepiece Wide-Field Binoculars F Star Field Observing Constellation
https://a.aliexpress.com/_m0Vfm6Y