Because combat in 4e is very tactical (moreso than previous editions of the game), it also has a tendency to take pretty long in real time (though your mileage may vary). This isn't always the case, but for tactically minded groups combat can almost be a game in and of itself (heck, it's one of the reasons why playtesting isn't as boring as it sounds in practice). A hefty length of time and a different strategic mindset separate you from the "roleplaying" aspects of the game during combat, which is great for creating a varied play experience but it could also be a bit jarring for the pacing of the story. For these reasons, the vast majority of major encounters should have some narrative significance.
In previous editions of D&D encounters with "wandering monsters" and dungeon crawls through miles of subterranean passages populated by all manner of random creatures were highly encouraged. From what I understand, combat in earlier editions was much more swift in real-time, though I only have experience with 3rd edition (indeed, the longest combat encounter I've experienced was in that edition, though save-or-die spells could make encounters equally swift and arguably inconsequential). If a fight ended up being uninteresting it didn't really matter, because it would probably be over quickly. Because of that, an encounter didn't need to have any particular importance because you were back to the story (or the next encounter, in more of a hack 'n slash style campaign) in no time. Because the default 4e encounter is assumed to be somewhat involved, this edition isn't quite as friendly toward that old style of play.
Ultimately, the take home message is that when designing an adventure, every encounter needs to have some kind of significance. Some encounters can be justified by being exceptionally interesting in and of themselves; monsters with cool powers, traps, terrain, puzzles, or engaging tactics can all make an encounter interesting enough that it doesn't matter if it doesn't have a strong link to the story. If you need "filler" encounters to give the players something to do as they make their way through the dungeon--an obstacle to their eventual goal--then they better be interesting. But every encounter can't be over-the-top and exception, because if that becomes the norm then it's no longer very exceptional! This is why most encounters should have a direct relevance to the adventure narrative.
But what does that mean, exactly? Obvious examples are encounters with the foes that the PCs set out to confront in the first place. This can be as epic as a final "boss battle" with the BBEG (big bad evil guy), or as simple as tracking down the group of bandits that have been harassing the local merchant caravans. Other ways to make combat relevant are to use the encounter to introduce NPCs (whether they're combatants or non-combatants, allies or enemies), to provide PCs with some kind of information, item or clue, to introduce a new conflict (perhaps a side quest), to foreshadow a future plot point, or to characterize an NPC (perhaps an NPC betrays the party, or maybe they die defending the party who will thereafter seek revenge). A more intricate encounter might present 2 or more decision points where the choices that the PCs make affects the story. Perhaps the choices are meant to reveal a character flaw, tempt, or highlight the chivalry of a PC. Any time you give the player the means to help develop his/her character more fully is time well spent. An encounter's relevance could also be as simple as providing a direct obstacle that stands between the party and their goal (i.e. they want to get into the Wizard's keep, but first they need to deal with his golems).
From a pure mechanical standpoint, encounters exist to challenge the party and put a strain on their resources. At its most basic, combat is the reason why characters have healing surges, encounter powers, and daily powers and any encounter will allow (or force, depending on how you look at it) the characters to use these resources. It's up to the DM and the players to use these mechanics to tell a story, and as the one designing encounters the DM shoulders most of this load. The story is what immerses the players in the world, and keeps them coming back for more. While a delve style game where players simply move through the map and kill a random assortment of monsters is certainly valid, it's simply not enough for some players (or groups). Those that care about story will enjoy the game much more if the story informs combat; an hour-long scuffle that turns out to be pointless can be an annoying distraction if it happens too much.