Because its stronger and easier to fit than a shackle and possible easier to source when you have 10mm chain as i have.
..........Of course I would not use any old swivel.
So lets look at the options with 10 mm chain, a fairly common size .....
I have the feeling that the average one in the chandlery
is too weak... can you really trust them.... the recomendation is to get one that comes tested with a swl of around 4 tons, then you must spread the last link to fit it. To spread the last link is not an easy task. To get a high quality tested shackel is not so easy... you probably have to order it over the internet
. (I used to know some sites but I forgot them... the Rocna
site has some links I seem to remember). Then you have to lockwire it on.
So even after you end up with a strong tested one off the internet
you still need to enlarge the end link to fit it.
(lets also not forget that shackles get an unhealthy side load pull when the pull is not straight just as a swivel does)
Well I dont know much about all those horrible swivels that everyone goes on about but there is no way that I would use any of them. The only swivel is a wasi powerball.. straight pull swl of about 8000kg I seem to remember and if you pull it with side load it will not spread the jaws, instead it will bend your anchor shank... well at least thats what happend when my anchor turned only half way and then the wind
blew > 40 kts.
You probably have to get it off the internet as well but one it arrives you just fit it and loctite the 2 special locking screws and then fill the locking screw hole with a bit of sikaflex and jobs done. No enlarging the last link at all.
If I could buy a tested shackle that was as strong as my wasi swivel (or at least stronger than the chain) and did not need to have the last chain link enlarged then I would swap to a shackle.
(If anyone knows of one... could they post a link to the site)
.....Well thats my opinion... Its not too far off is it???.