As a buyer you can research home warranty companies and choose the company and coverage that you want. It's important to see what the company's policy is on items that are important to you.
Anyone can purchase a home warranty on any home at any time. Most companies charge a $55-$60 service call fee anytime they come out and they will either fix or replace anything that is covered and not functioning. **Be sure you call the home warranty company BEFORE you call anyone else. If you are having a plumbing problem and you call a plumber to come fix it and then call the home warranty company they will not cover it. They have a list of approved vendors and they must schedule them to come and insure that no one else has serviced it.
Also as your policy comes up for renewal the warranty company will send you a notice to renew for a higher price than the original policy. Be sure to shop companies and get the best price & coverage.
So are they worth it or not? I had a client once who was a single mom and so excited to purchase her first home. Within a week of moving in to her home in July her air conditioner went out. Thankfully she had a home warranty, called the company, tech came out and said the unit was shot, the company replaced it and she only paid $55 service call fee. Instead of several thousand dollars. So I'd say that it's worth it!