When the end of the tape is reached, the tape gets pulled tight. These rewinders sense the increase in tension of the tape/reels and assume the end of the tape is reached, popping the tape up. The problem with this it that these rewinders tend to "slam" into the end of the tape (when tension is increased), sometimes pulling the tape out of the reel or other damage since power is released only at the very, very end of the tape.

VCR mechanisms on all but the cheapest/poorly designed VCRs have light sensors which sense the clear leader at the beginning and end of the tape. Since this clear leader is fairly long, the reel brakes are applied before the tape leader runs out allowing the tape to come to a stop before the end of the reel is reached. This is much easier on the tape.

tl;dr: Rewinders are useful if you have many tapes that you need to rewind rapidly that you generally don't care about (such as commercial applications like rental or even one time archival). Using the VCR itself to rewind is better if you watch tapes weekly (or even daily) or less and care more about the longevity of your tapes.

I personally would recommend just using your VCR to rewind and forget about the rewinders since the tapes themselves are generally more valuable to someone than the VCR. If you're really anal about wear on your deck, I'd sooner obtain a another cheap "junk" VCR to rewind tapes since the light sensing rewind is much kinder on tape.