These are often called ball stretchers, they separate your balls from your shaft and make you shaft look longer, they appear to firm up and shape your balls pushing them forwards and presenting them.
Personally I preffer the Oxballs Squeeze ballstretcher, available from Uberkinky in the UK:
https://uberkinky.com/products/oxbal...tretcher-steel
and direct from Oxballs in the USA:
https://oxballs.com/shop/ols/product...etcher-oxballs
I would tend not to wear them under speedos as they do show through the fabric, not at a public swimming pool or busy beach anyway.