Why did my grandfather-in-law salt his beer?
I can think of several reasons why you might salt beer:


Salt is a natural flavor enhancer, so you'd be able to taste the hops and malt more
Salt reduces perceived bitterness, so overly hopped beer would taste less bitter
The salt crystals may nucleate bubble formation, giving the beer more head (briefly)


I've heard of it being done before, but never with good beer, only low quality swill.  But then, since there were more quality problems with cheap brews at the time, this was probably a lot more common.