DC relays also have a tendance to "magnetize" and get either sticky or sluggish after long, long periods of being continually energized. The magnet may be to help prevent this by repeling the armature back to the resting (N.C.) position.
I have seen this happen in industrial control circuits.
That is said to be a problem especially when running a.c. relays off DC - something I have done for years. Usually, a 110v a.c. relay runs perfectly off 24-30 vdc, and much more quietly. That has never been a problem with mine. The only relay I ever had to magnetically stick, was designed for DC: a relay salvaged from an ARC-5.
Of course, I had run that relay in my transmitter over a period of several times the duration of the War. I'm sure they never designed a life expectancy of 20+ years into WWII anything. Wouldn't the engineers at companies that designed and built BC-610s, BC-348s and ARC-5s, be smiling in their graves if only they knew that some of their handiwork would still be in daily operation 70 years later? I just replaced my relay with a better one, not from an ARC-5, with no problems since.
A simple solution if DC (or DC-operated AC) relays become sticky is to reverse the polarity of the control voltage. Maybe even install a DPDT switch in the control line and periodically flip the polarity as a measure of routine maintenance (
do this only if there are no diodes across any of the relay coils). If you want to get fancy, design a simple logic circuit that automatically flips the polarity every other activation. But as I said, except for the ARC-5 relay, some of my relays (both AC and DC) have been running off 24 vdc since the 1970s or earlier, and they have never become sticky.