I've been looking around the used market at fish finders and see a lot of people listing the hours on the unit. Is this number of any relevance?

Do they spontaneously combust at a certain number, I don't get it. 

As far as I know, the numbers just show the number of hours the unit has been run.  I would imagine the number would be more important to an individual buying a used unit. It would keep the seller honest, to a point. You wouldn't be able to sell a used unit with 200 hours on it as new.  Same as outboards...and my truck.

Understandable. You truck probably has a warranty like xx miles or 5 years whatever comes first. Is this an issue with fish finders? It would suck to have one bust after 300 hours and have a manufacturer say the warranty is good till 285.  

I guess I don't get it. I never sold/bought a tv, phone or any other electronics and indicated the hours of use on it. 

The warranty on electronics is usually based on time from the purchase date not hours on the unit.  The hours are just something they included in the software.  You could ask the manufacturer why they include it but CS may not know.  I'm sure the techs and engineers have a reason for having the info.

Advertising a sonar as a "low hours" unit is probably a good thing for marketing and perhaps to foster buyer confidence. Even if the unit is higher hours, the seller might still advertise that as a means of good faith / disclosure so the potential buyer knows what they are getting in terms of definable prior usage. Using terms like "used only X seasons" is meaningless, just as it is for other equipment like reels. One persons "season" might be 300 days on the water - another persons "season" might be 30 days on the water.

I like having the hours feature. I record the hours on my sonar at the beginning and end of each season to calculate how much time I was in the boat that year. I usually always have the sonar turned on but sometimes turn it off in skinny water, etc. so the hours recorded for a season are actually a little low from the actual time-on-water. Still, it's an interesting factiod that I record.

For instance, the total time on my current HB 798 is 1428 hours with 531 hours of that recorded during the 2015 season. Interesting to me...