It's an insurance policy and how confident you feel about the quality and reliability of the system you install. You are paying for the labor usually after the first year.

The two major components you are guarding against is the outside condenser compressor and the inside furnace/air handler's blower motor especially the var speed variety. The part is covered but not the labor unless you have the extended labor warranty.

I am not a fan of these 3rd party ext warranty companies. The majors have dropped their manufacturer ext warranty which were more reliable than the 3rd party companies of which many have terrible reputations. The price is generally no more than 10% of a complete installed system.

Proceed carefully and make sure you understand what is covered, under what circumstances, as well as all exclusions. As well as your obligation for regular PMs.

IMO