one way to think about it is that a bread knife is basically a saw. what it does is entirely different from what a (good) knife does. that idea's lost when so many people use serrated knives for this and that (note: a "tomato knife"?! oh my god), and when so many super-duper tv knives are able to dupe people into buying what are actually saws [made out of truly embarrassing metal, but/because it doesn't matter], but if you were to look close, you would see that what a good knife does is very different from what a saw does. it's not really even true to think of a good // very-well-sharpened knife as a saw on a microscopic scale, weirdly enough, because even though it's true that some "tooth" is very helpful to avoid skating over a super-smooth surface, like tomato skin, you're not really still sawing with a hyper-sharpened and polished edge that is not "toothy", but it will still cut like a terror; it's not ripping and tearing the surface, it is -- in a weird way -- parting the material it's coming into contact with, left and right. (this is very much related to the geometry of the grind, "thinness behind the edge", etc, no less than the angle and finishing of the edge itself, and helps in thinking about what that metal is doing as it heads into the material-to-be-cut -- and parts it, left and right.) once in a while, when you meet a terrifyingly sharp, you know what this feeling is; i can't sharpen a knife worth a damn, but three or four times i've been lucky enough to handle such a knife and it's... absurd: you know that you can't even really touch it to something without it being cut, and you can sometimes actually cut yourself without feeling it, because it hasn't sawed/ripped into your skin, it's just gone in. same way, it will _actually_ feel like it's _melting_ the material it's passing through. there was a reason somebody felt something like that one day and came up with "hot knife through butter". in a roundabout way, there's a mind-bending moment in some commentary by richard feynman about this idea, somewhere in a little question-and-answer video, mainly about how people don't really think about what it means to cut or part or push through some material with some other material -- nor the opposite, e.g., when one surface pushes back on an object, like why you don't fall through the chair you're sitting on. in a totally different way but on the same point, america's test kitchen makes this point in a very nice and practical way just in terms of its recommendations for steak knives: briefly put, never buy a serrated steak knife. they are cheap and you don't notice their getting dull, etc, etc, but they are ripping and tearing -- sawing -- your food, and that's pretty terrible for steaks most especially; you should always, if you care about not thrashing what you're cutting but actually just _cutting_ it, get straight-edge, not serrated, steak knives, even though, yes, you'll very quickly notice their getting dull (sadly, on ceramic; it's a horrible feeling, isn't it?), but if you can keep them sharp, they'll actually cut what you're trying to cut, not shred it. ... or just use cheap paring knives and keep sharpening them.
the serrated bread knife's just being a saw also means, as others have said, that the "grid" of the bulk of the blade is mostly irrelevant; the serrations are creating saw teeth, teeth for biting into and tearing the material, and as long as these saw teeth are basically still there and not _too_ rounded, it'll still saw the bread -- and, more importantly, will succeed in that first step of not skating over the hard crust of the bread. this is related to a crazy japanese knife/saw company's "solution" to the forever-sharp knife dream: if you look really close, you'd see the micro structure of the material was such that, as the material wore down, it constantly moved on to expose newer layers of micro serrations, just like rows and rows of sharks' teeth. ... but... "that's not a knife!", that's a saw. one thing about getting really great (actual-)knives is to experience how they can cut (most) bread. it's like a revelation to actually cut bread instead of saw bread, and most good and super-sharp knives can handle most medium-crust-y breads just fine [except you should obviously avoid crusts if you have a fragile/chippy knife; i've seen people comment they don't even like some knives' edges to scrape against the crust of a steak or fish skin]. in fact, i think the most times i hear "wow, that's a sharp knife" is when they've grabbed even a modest victorinox -- but one that's new or very well sharpened -- and cut some bread with it when a bread knife wasn't handy: you feel how it doesn't crush/compress the bread, like a dull knife would, and you also feel how it's not sawing the bread, like a bread knife would. obviously, the situation's different if you're dealing with a very hard-crusty bread. (... but i for one don't often seek out super-hard-crust-y bread, mostly for the same reason i don't usually race toward opportunities to have my hard palate ripped to shreds by shards of broken glass.) along these lines, one of the coolest demonstrations of a knife's sharpness (and you can catch this once in a while in a youtube video showing off a recent purchase or the results of an afternoon spent sharpening) is how it cuts through bread without compressing it at all. it just glides through and the slices fall over. i find this even more compelling than the usual tomato demonstration.