If you regularly tie tzitzit yourself, you know it's very rare for the tzitzit to come out the same length. Or you might be among the few who adhere to halacha that says attractive tzitzit (noy tzitzit) are wound and knotted for one-third of the length (the gid) and hang loose for two-thirds (anaf).
So how do you trim the tzitzit strings? Once our own zirconium oxide tzitzit cutting scissors broke, and we needed a an interim solution until they could be replaced. So during an outing to Jerusalem I asked my sons to find shards of glass. (This part is easier if you live in or near a bad neighborhood.) Each one came to me with his shard, and I chose one with a nice sharp cutting edge. For about a week that's what I used to cut tzitzit. It was about three times the effort of ceramic scissors and the results were far inferior.
Once someone asked me why this product came into the world. Surely the demand among Jews to avoid trimming scissors with metal implements is not enough of a market force to stir manufacturers into action.
Small ceramic scissors are sometimes used by fisherman to cut fishing line, because a wet tackle box makes their metal scissor rust. Electricians may have these scissors among their tools, because they don't conduct electricity. And cooks may keep a pair on hand in the kitchen, because they don't impart any taste to food (e.g. when cutting lettuce), they never need sharpening and the smooth surfaces are easy to clean.
For tzitzit, they are not only very helpful for trimming when you finish tying, but also when the tzitzit start to fray a bit at the tips, instead of rushing to replace them you can snip off half an inch and they look like new.