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If direct then yes flushing it is very beneficial,because you are removing the sediment brick that lies between the flame,metal and water which slows down the heating transferring process,which wastes fuel and shortens vessel life. As you know hard water and electrolysis is a by product of direct fire units.most tanks have the common anode rod made of zink,mag,aluminium and other metals,some newer ones are called impressed current rods, they are charged to attract negatively charged metels that are in the water and are more easily picked up during electrolysis,this type of ananode needs to be changed out more frequently, but these are mostly found in commercial applications.
Another type is titanium and these are directly charged and never need replacing, ever.there are a couple more.


Anyway the ananode rod will help the prevent the deteiration of the glass/ceramic lining of the tank,but flushing is being proactive,which is helpful on your wallet,but you can go one step further and de scale it which will eat away the harder sediments from the years you didn't flush it.
These kits can be purchased at FWWebb,Grainger and other plumbing supply houses.


Where as indirect fire will have far less debris/sediment on the bottom because there is no direct flame to metal and electrolysis will lower.
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