I use them to buy DVDs. There are always at least 10-20 of any dvd I'm looking for. If I had to bid on eBay I'd be locked into bidding on one item then waiting to see if I won it.
With a bidgroup I can snipe all 20 items of the same dvd and sit back and wait to win one at the price I want.
Usually I figure that price out by doing a completed item search and seeing what the lowest price that item has sold for in the past 30 days. I'll take the 5 or so lowest prices people have won it for and then bid in that area. I usually save in the area of $2 and that's just on a $10-12 dvd. So it's a good example of how bid groups and sniping can save you time, and a little money on even a relatively inexpensive item.
Before bid groups I'd drive myself crazy bidding one at a time on dvd after dvd until I won one. And even then it could have taken me a week or two to do it and I usually tended to creep my bids up by 50 cents to a dollar towards then. I had at one point pretty much given up on buying them on eBay. It just wasn't worth the hassle to save a few bucks when I could buy it at walmart. But now I save a few bucks and it's not really much of a hassle.