Why Do I Need an Electric Fence Tester?

Electric fence voltmeter led (i think) not working anymore, if anyone can tell me what kind of led they are and how the whole circuit works because i don't really understand it

Fence Tester – R18 Fieldguard Electric Fencing

Electric Fence Tester Wireless 8KV

Electric Fence Testers

Gallagher Multi- Function Electric Fence Tester G50405 / Free Shipping

Easily check electric fence voltage with this simple light-up tester. Measure 2000V-8000V.

5 Light Fence Tester

エレクトリカルテスター - 工具/メンテナンス

Nemtek Electric Fence Tester

エレクトリカルテスター - 工具/メンテナンス

Electric Fence Tester T-11, Wireless 5-Light LED Tester

Electric Fence Testers