So much misinformation gets dumped around about hazards because, guess what, the legality is different in every state. Here's the deal in Maryland: source.

Warning lamps - Permitted.- Any vehicle may be equipped with lamps used for the purpose of warning the drivers of other vehicles of the presence of a vehicular traffic hazard requiring the exercise of unusual care in approaching, overtaking, or passing, and, when so equipped, may display the warning in addition to any other warning signals required by the Maryland Vehicle Law.

In short, hazards should be used to say "hey you need to watch out and be careful about driving near me, something might be up!" Hazards may be used while stopped to mean "hey please don't run into my unexpectedly stopped ass!" Hazards may also be used if you're attempting to pull over or exit the highway during an emergency(for example, your tire pressure alarm is going off and you're in the left lane), as a means to warn other drivers that you're having an emergency and to please stay back a moment. They can also, under MD law, be used as an additional warning light in conditions such as heavy fog or snow, even while your car is moving. Illegal in some states, but not illegal in MD.

Using them for a funeral procession doesn't seem to be supported by law, however. I know everybody does it. They all also run red lights during the procession, doesn't make it legal. No cop is going to ticket someone over that at a funeral though, so people just keep doing it without repercussions.