It's for fish that get big and powerful. When they thrash about (sometimes for no apparent reason) they can break the heater. Tronic sales heater covers separately for their heaters or you can make your own. I've used a pvc pipe that I've drilled holes in to let the water flow through it. The disadvantage of having covers on them is that it may distort the temperature mech within the heater unit. So your water may not get as warm as you hope without tweaking the heater temp alittle bit.
Having heater gaurds for rays are mainly for BURNS.....Rays love to swim and touch the glass, and will touch the heaters...and they will get burnt.....Not good for them....
They are not so much for larger fish or breaking them.....
I made heater guards for mine. They're just a mesh tube a little longer than the heater itself. I put powerheads on the bottom (pointing upwards) to circulate water over them.