[Offwego]

A neighbour of mine collects bags of these crushes them and adds barrowfulls to his garden
Apart from the obvious slug deterrent via the gritty sharp edges are there any other benefits?

He also adds seaweed by the  barrowfulls and grows great crops.
I've always known that seaweed was good as my father who was a very good gardener used to send me on my bike to bring backs 3 sacks a time when I was about 12 cycle down push the bike back with the sacks draped over the handle bar and centre bar 😅 6 or7 trips at the start of the season which he used to dig in

[Deborah1]

Well, I don't have any scientific evidence to back up my doings, but I've been bringing back bags of washed up seaweed, bags of shells (plus the shells of the mussels I collect for me and the cat), and a bottle of seawater almost every week for the garden for the past ten years or so.

The seaweed gets spread around as a mulch. (I do eat some of the best stuff and dry some too.) The shells get scattered - crushed eventually as they get worked in - and I've always presumed they give a kind of slow release lime to what is a naturally very acidic soil. (I collect cuttlefish bones too).

The seawater gets diluted 1:10 and watered over the garden with a watering can fairly systematically, but initially it's for the asparagus and carrots...
I like to think it's doing some good, but it definitely doesn't do any harm!

[Aunt Sally]

There are restrictions on removing "stuff" from the sea shore. 
You have to have permission from the owner of the shore.  It may be a council, a private owner, the National Trust or the Crown.

Please be sure you do not breaking the law!