This is designed to prevent the easy removal of cheap cycle computer from bicycle. This means you should be able to leave the device on your bicycle and not have to worry about removing it when left unattended.

It works by preventing the removal button from being pressed as well as preventing the unit from sliding out of the mount. The only way to remove the speedometer is to cut the cable ties securing the mount to the bicycle

Although it will not stop the most determined of thieves it is designed to stop the casual passer by or evil doer from inconveniencing the owner, given that the device it is protecting is of low value it would be unlikely that they would carry the required tools or exert the effort to required to remove it with this device in place.

See photo to see device as it is a generic device widely available under different names/brands, here is a link of one such device

https://www.aliexpress.com/item/1005003198388904.html