It's important to bear in mind that All Quiet on the Western Front was written in 1928, five years before the Nazis came to power. That said, the book remains a trenchant critique of the militarism and extreme nationalism which the Nazis enthusiastically endorsed. This is why the book was banned—and copies of it publicly burned—after the Nazis took over.
Strictly speaking, boots are not a motif in the story but rather a symbol. And as a symbol, they represent, among other things, the fetishizing of war. When you fetishize an object, you devote an excessive attachment to it. And that's precisely what Müller and the hospital orderlies do in relation to Kemmerich's boots. They almost venerate them as objects of respect, paying more attention to them than to the man wearing them.
In this regard, Kemmerich's boots symbolize something else: the cheapness of human life during war time. Tens of thousands of young men are being slaughtered on the battlefield each and every day of this seemingly never-ending conflict. Yet, the warped militaristic values that the War encourages attach greater importance to the objects of war, such as army boots, than to the lives of those sacrificed to the insatiable demands of the German war machine.