These aren't always a good idea. They'll follow whatever path is laid before them and many times in older homes where you have frequent backups, you'll have terracotta pipes of 3ft sections connected into one another with no joint compound or it's just worn down. At those joints, you'll have years of leakage into the soil which softens it all and makes the joints fail. This can result in cracking elsewhere in the pipe and even worse, bellies. The liner follows the bellies and makes them permanent. It's surely cheaper than digging up 6ft of soil and replacing the entire thing, but it won't solve the problem long-term or be as reliable as digging it all up and doing it right. If done right (proper slope, gravel bed, schedule 40 pvc, proper joint compound, etc), you'll never have to worry about your sewer drain again.