Those third-party clients have some essential, basic functionality that the official ones for some reason lack. Signal-cli allows registering from desktop without any smartphone, Molly allows an arbitrary Socks proxy instead of being limited to just Signal's own proxy solution, tying a desktop client with a link instead of scanning a QR code (thus allowing easy registration from an Android VM), and maybe most importantly for some - Notifications not relying on Google (Molly-Socket allows it to use UnifiedPush).