For payed ferreting they are a must. You need the job to be quick and slick as time is money so as soon as You think the ferret is holed up you need to be able to dig it out and move on.
I did a job on a big graveyard at the end of last year and had a mark on a ferret that was obviously stuck and didn’t show. Blocked up the holes set a catch alive trap and went back the next day. No ferret so we had to dig 6 foot to get her out ... without the collar she would have had a slow death stuck underground .......