I'm guessing it how you link the together can you explain why you would and how it helps

I'm looking at a 898 HB for my bow and a 5 in for the dash but what I want to know is how it would help me if it would

Lets you share way points, GPS, etc between two fish finders.  Allows you to only need one transducer too.

 

http://www.humminbird.com/leading_innovation/fishing-systems.aspx

If you want an Ethenet connectable 5" unit you had better do your shopping soon. All of them are being discontinued for the 2014 model year.

 

Ethernet shows the waypoints of one unit on the other unit in a different color, it is does not write the data to the other unit's memory. It shares accessories such as a GPS antenna, transducers. temp sensors and it is the connection for several accessories like RADAR, 360, and iPilot Link.

 

Both 898 models are discontinued and being replaced by the 899ci HD SI Combo. Some good closeout sales on those units.