Both types are designed to help avoid line twist and are tied on about 3 m to 4 m above the squid jig. Or, in the case of the small one, for sabiki fishing too, and for leatherjacket fishing.

The larger sinker has flat sides to stop rolling around on the deck of a boat. Swivels are black to reduce the chances of e.g. mackerel chasing them.
Heavier ones are also on the market, shown here are the lightest.

From Yamaria