The reason to sell war bonds in a fiat money world, at a base level, is that you want to devote some resources to the war effort, and therefore those resources are not available to produce consumer goods.
Now there are two options. Either the consumers have just as much money chasing a smaller amount of goods, leading to inflation, or you need to reduce the amount of money consumers have. Selling bonds is a mechanism for doing the latter. The end result is the same: the war effort means lower living standards, whether through inflation or through people voluntarily forgoing consumption by purchasing war bonds; the latter happens to make people less upset and likely has different distributional characteristics, so is probably a better idea.
Note that there are various details like whether your economy is at capacity (and if it's not, maybe you can do the war effort production _and_ still produce the goods consumers want by expanding overall production). And there can be various different mechanisms which lead to inflation in practice, ranging from "printing money" to pay for the cost of the war effort production, thus bidding up prices in general, to establishing rationing and a corresponding black market with high prices.
In a non-fiat-money world, there can be other reasons to do war bonds, of course, including "getting some of whatever counts as money", since the government does not control the supply of that.