As already mentioned drums can wear unevenly or warp and become out of round. The drum often wears most closest to the mounting face because the brake dust & debris can't get out of the drum. 
There are three holes around the edge of the drum that need to be cleaned out, these help shed water & debris.
If they are grooved, new shoes take a long time to bed in and get full contact. If you think about it a grooved drum will actually have a bigger surface area and as long as the shoes fit in the grooves exactly then better braking......
Often they are still 'round' but because they have worn unevenly and/or have grooves a  'brake place' will insist they need machining. So far, touch wood, all  of mine have been worn but not out of round so I've never had to have  any machined but I have scraped the wear lip off so the drums go on  & off the hub easier.
Beware Ebay purchases, **itpart etc. Cast iron needs to be good quality and cooled down evenly otherwise you may get problems. A good brake place can track down 'name brand' drums but they are more expensive. I've noticed with the cheaper drums that instead of having three countersunk holes to hold the drum in place they often only machine one. Should be OK, only really comes into play if a wheel comes off and you want the drum to stay in place (god forbid).
Last quote I got (trade) was $60 for LWB drums from Europe (15GBP from Paddocks but freight would make it up to close to $60).
You really need to find an 'old school' brake place but they are getting harder to find. Father-in-Law worked for Neville Pearce Brakes in Hallam but he sold the business some years ago then the new owner shut the VIC operation down. They had drums cast for vintage & veteran vehicles, repaired shoes, machined shoes to match drums, worked on vintage race vehicles etc. etc. Another old place in Dandenong closed a year or so ago.
Colin
				
			 
			
		 
			
				
			
			
				'56 Series 1 with homemade welder
'65 Series IIa Dormobile
'70 SIIa GS
'76 SIII 88" (Isuzu C240)
'81 SIII FFR
'95 Defender Tanami
Motorcycles :-
Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C, Suzuki SV650
			
			
		 
	
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