Island drains deal with sand, and older island drains deal with sand plus decades of narrowing.
Sand settles rather than washing through
Every shower in a beach house takes in sand over a season. It is heavy, so it collects in traps and low spots rather than carrying on out. Jetting flushes it properly where a cable only makes a channel through it.
Older pipe narrows
Cast iron in the original cottages builds up a layer inside over the decades, so the working diameter is a fraction of what it was. Combined with sand, that is why an old island drain can slow down without anyone putting anything unusual down it.
Outdoor shower drains
These take the most sand of anything on the property and are the most commonly forgotten. Worth including whenever anything else is being cleared.
Camera first on old pipe
If your house predates the war and a drain has backed up more than once, footage tells you whether you are cleaning a pipe or postponing replacing it. Old cast iron that has thinned may also not take jetting, and that is worth knowing before anyone starts.