I am going to do something really stoopid and tell you that I completely disagree with Karl on this one, Frank.
Karl, please accept my apology up front.
I LOVE the magnet. I have one that has a little built in handle, a very strong magnet.
I do NOT use this magnet when grinding the bevel or hollow grind, when doing stock removal.
However, it is most useful when you want to smooth out barstock that has a rough finish, such as taking a blade and making nice smooth sides prior to grinding the bevel. You can hold with the magnet, and pass over a grinding wheel from one end to the other, and get a good smooth finish.
ALSO, as the blade gets hot, no problem, as the magnet is holding it. You are going to treat and temper later anyway. So it saves a little "dipping" time, but more importantly, more comfort.
ALSO, you can hold small parts when grinding or polishing, such as guards. Parts that are hard to hold unless you grow your fingernails.
I put the magnet away when grinding the bevel, and put on a tight fitting mechanics gloves, just in case I get too aggressive. You can feel heat through them and not run the blade too hot when that is important.
Dan