It's not NEEDED, but I gotta be honest, it's a very handy and inexpensive tool.
For example, I thought I had a problem with my P-38 build - one of the aileron servos wasn't working properly, and I thought I had a bad connection or a short in the extension I was using. The servo tester I have has 3 settings - a dial where I can move the servo in range, set it to center, and one that goes back and forth continuously. The back and forth movement worked out for me; I was able to have it run, and wiggle the wire - turns out the extension wasn't connected fully, and was loose. A servo lead lock helped keep the connection together and it didn't have any further shorts after that.
I MIGHT have been able to diagnose with a transmitter and receiver, but it would have taken significantly longer, trying to keep it wiggling and moving to see if it shorted out while in motion. Is it a NECESSARY tool? No. but it does help in instances like this.