Sacrificial anode is there to protect the metal parts of the tank from corrosion. The anode corrodes preferentially and protects the brass, copper and steel parts until it is gone. Made from a magnesium/zinc alloy it will last around 5 years on a soft water supply, up to 15 in a hard water area. The current generation of tanks have a steel shell with a glass coating on them for protection. No more copper tank ( at the scrap price of copper no wonder) and very little brass fittings used on them. I replaced a valve on one 2 months ago, and stripped the tank and cleaned it while doing the draining. Inside there was a thick layer of sludge from the incoming water, leaving a 3cm layer at the bottom of the tank that drained off looking like tea. That was the settlings from what got past the 100 micron inlet filter on that line set. Glass lining and anode were fine, not surprising as it is only 2 years old. Replaced the filter cartridge as well, with a 5 micron unit. Old one looked like a brown blob from what was in the supply line.
As to heater efficiency the best improvement is to add insulation to the piping to and from the tank for the first meter, that will drop your standing loss by at least 50%, and adding extra insulation to the tank ( I used a roll of wall insulation wadding, light green fibre made from recycled PET bottles) drops it a further 30%. Made a big difference in the electricity bill.