Itemizing effectively is important
Itemizing effectively grants you value without requiring any mechanical or decision-making skills beyond the act of itemizing itself. You can win and lose games based upon item choice alone. For example, the decision for a marksman to build armor penetration against a team comp that's invested heavily in armor is important, as if they just stuck to comfort-building lethality (flat armor pen) over % armor pen, they're actively shooting themselves in the foot, as their damage will be far less effective against high-armor targets. The result of this would be that the marksman would be effectively facerolled by tanks through base damage alone.
Your comfort and experience matter too, though
Consider your Soraka example. You built Warmogs presumably because you're quite used to playing that way, I imagine. Sitting back and constantly healing your allies while Warmogs replaces the health you sacrifice with Soraka's W is an effective strategy. What really does cut into that is, as you pointed out, a very good Zed player who knows that you're a fairly squishy target without any armor, and removes you from the equation by all-inning you. Would replacing Warmogs with Zhonya's in this instance stop that from happening? Maybe once in awhile if you're not caught out without teammates to assist you. If you're playing the game out and you're determining that you don't need to defend yourself using Zhonya's active stasis or its armor values, then stick to what you feel works and will benefit your team the most.
You did what you felt was right. You felt that you would be more effective to your team by boosting your shield and heal power. Perhaps that was because you felt Zed wasn't a big threat, or something else made you feel comfortable doing that. Whatever it was, try and quantify that as you play more, and tailor your build to the game at hand. There's no one build that will defeat all others in every single game, but there are specific choices you can make that can make a big difference in each individual game you play.
In summary: Think before you build, maximizing your personal effectiveness
Consider your enemies and allies with each item you purchase, and maximize your effectiveness within that realm. Certain champions have a set of core items they require to get off the ground - for example, Yone and Yasuo require a crit item (typically their Mythic item) and Infinity Edge. Build those first, of course, but beyond that, consider each further option carefully. If you're against heavy magic damage and poke, consider a magic resist item like Wit's End. If you're against heavy physical damage, consider a defensive option like Death's Dance. If you're profoundly ahead of the enemy team, consider building more damage items to accelerate your lead that may also carry passives to cover what you feel may be a later weakness.
It's generally not advisable to brainlessly build a preset build unless that particular combination is really meta at the moment, regardless of your rank/skill level. You will inevitably find yourself flailing against various champions and team compositions if you don't tailor your build at least a little bit per-game.
Riot has done a pretty decent job (at the time of writing) making item choice somewhat varied for each champion. There are some exceptions, such as mage items being pretty bland, but there are very few brainless builds in the meta at the moment. Take build guides you read online as a framework, not a strict set-in-stone contract, and don't be afraid to experiment a little!
