On a small island lot, being able to repair a sewer line without digging is often the difference between a manageable job and a very disruptive one.
There is usually nowhere to dig
Folly lots are narrow, and the line generally runs under parking, decking, a walkway or right along a boundary. Add a high water table and a trench that fills with water as fast as you dig it, and open excavation becomes genuinely awkward. Lining works through existing access points instead.
Good for older cottages
Where an original beach cottage still has its first sewer line, lining creates a new pipe inside the old one and seals the cracked joints that were letting groundwater and sand in. That addresses the silting problem as well as the leaking.
Newer elevated homes
If your house was rebuilt after Hugo or later, the sewer line is much newer and lining is unlikely to be what you need. A blockage there is far more likely sand or something that went down the drain. Ask to see the camera footage before agreeing to anything.
When it cannot be done
If a line has collapsed or lost its fall so badly that it holds water along its length, there is nothing sound to line against. Better you hear that after we look than partway through.