Their primary function is to support/align the cartridge in the magazine. You'll notice that the ribs are in slightly different locations and are different depths depending on what the magazine is for (i.e. 204/223 v. 22-250/243/7mm-08/308 mags). You'll note the ribs on say a WSM magazine are fairly shallow since that's the largest diameter case the magazine design was intended to be used with.
As for additional strength, naturally the ribs will add some but it's merely a side effect that really serves no purpose as there is no reason additional strength would be needed.
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