Okay, consider this scenario: You've been living in a house for 15 years. The tub is backed up. The garage utility sink is backed up.
This has happened before. One time, the backup was really bad, so you had a couple pipes replaced. The pipes under the house are an odd collection, with a couple of difficult joints which cause backups more frequently. You know where all the clean-outs are in the house.
Now, you call a plumber to snake the lines. He arrives two days later. You explain what is happening. He seems to understand, says he'll take 10 minutes, and begins to snake the plumbing from tub.
After 20 minutes, he's still going.
So, you walk into the bathroom, and offer, "Hey, so, the lines are kinda tricky in this house. The best clean-out is on the roof. Would you like to see where the other ones are?"
The plumber turns to you and responds, "Are you a plumber?"
You answer, "No."