With today's components it just doesn't seem to be necessary as long as your primer pockets are tight.
Not that many decades back, depending on who made your components the quality control wasn't as good as it is today. Many of us almost exclusively used surplus military brass. A lot of it had crimped primers as much of it still does today. The old CIL components were always good with Dominion brass but it was iffy when mixed with US/European components. Where I lived, primer types were limited. If I made a trip to the US, I would stock up on powder, primers, bullets and brass.
Usually I would pick up primers by the box of 5000 and usually at least ten or more of those. Then a half dozen 50 pound kegs of surplus powders and kegs of bulk bullets. We didn't worry much about whether primers were standard or magnum for the simple reason magnum primers were never on sale and may places just didn't stock them.
There were very few places in Canada where these components were readily available without a several month wait. I knew fellows that would order from Sydney I Robinson, International Firearms, including myself and often the orders would take anywhere from two weeks to three months to arrive.
Anyway, the above is to explain how the practice came into being. Some military ammo is still sealed with lacquer. But that is for extremes we as hunters/target shooters will never see. With the specs and tolerances being so strictly adhered to in today's manufacturing processes using sealants is IMHO a tedious step that I quite happily eliminated about 20 years ago.