Is it really needed or they can be synced without it?

I bought a Uni-sync one, got it ordered from america. It didn't work very well. I found the readings it gave jumped around too much to be accurate. I find I can tune my webers better by ear, feel and taking a plug lead off one at a time to see the difference.

But having said that a lot of people like them and seem to use them with success, so maybe I just got a crap one?

But to answer your question, you don't need one. And I don't use one.

i use one for my twin kadrons. it turns a long process into a 5 minute job!!!!!!

Learn to do it by ear - it takes longer, but you'll do a better job in the long run (sync tools really only set up your idle, carbs should be synced throughout their range.) Also, vacuum-type sync tools don't work too well if you have a radical cam.

Even though I don't have a multi carb VW I have one in my kit as a result of years of owning MG's.
I got quite good at using a length of garden hose stuck in my earhole & listening to them suck, but, I found that even a pair of SU's I had set up to run well were still off when I checked with the Uni-sync. Then there's the time element too.
I beg to differ that you can only use them at idle. By screwing down the restrictor, you can measure higher air flows. Don't know about max revs though!!