Yup see this is exactly what I was worried about. Luckily I don't think the website is making claims anywhere for what its purpose is, I just wish it'd explicitly say somewhere "yo if you're a noob, this is not the place to come to learn how to build a workout plan. It's an index and reference for once you've already put exercises on a plan."

In regards to your "why," it's a pretty broad question to answer. Why should you do weight training? It's good for your health, usually, ask your doctor for if that's true for you specifically (usually the answer is yes). As in, studies show health benefits unrelated to the actual benefits of bigger muscles and greater strength:

1. Improves heart health 1, 2, 3, 4 (sorry too lazy to properly format cites)

2. Lowers risk of diabetes, good for managing blood sugar 5 6 7 8

3. Positive psychological effects (outside of just feeling better about your appearance, it literally affects your brain chemistry) 9 10 11 12

Consider also some of the more "common sense" or "obvious" side effects of simply being stronger: reduces your risk of injury, increases your bone strength, boosts your self esteem (even if your culture doesn't value macho muscly people, simply being stronger can be a self esteem boost), and, being stronger is just kinda fun and nice and convenient. Consider: one trip from car to the house with grocery bags, or, picking up your dog / kid / friends more easily. Even when I reach up for a big bag of flour and feel my muscles at work, I find some amount of satisfaction.

If you mean "why should I work a certain muscle group," the answers are all over the place, and really situational. Someone might be working a specific muscle group because of an injury and are doing restrengthening or creating greater strength in a way to prevent future injury, or, because they need a rebalancing for compound exercises (i.e. their abdominal strength is a blocker for greater deadlift weight), or, aesthetics (bicep curls, all day, every day, or, the more modern variation: waaaay too many squats).

If you're "someone who knows almost nothing about muscle building," and are also interested in it, I recommend a workout plan that involves compound exercises. Start with your why so you have a goal in mind to drive you, i.e., I wanna be able to carry my kid for longer, I want to increase my quality of life, I want to appear more aesthetic, I want to feel more confident when hiking with weight, whatever, pick something. Then, consider your equipment situation: do you have access to a gym and the time to go there? If so, barbell compound exercises are worth investigating. If not, bodyweight exercises (these are great to do with barbell exercises if you have the time, I really think everyone should be doing at least a little bit of bodyweight exercises every few days).

From there hit the forums:

https://thefitness.wiki/getting-started-with-fitness/

https://thefitness.wiki/muscle-building-101/

https://thefitness.wiki/routines/

https://www.reddit.com/r/Fitness/

If you're looking for something to do literally right now, and you haven't had a doctor advise against physical exercises for whatever reason, I guess get on the ground next to your chair and see how many pushups you can do? Here, the muscle wiki is useful for this: https://musclewiki.com/bodyweight/male/chest/push-up though it might help to watch a youtube video as well, I really love the "Hybrid Calisthenics" channel https://www.youtube.com/watch?v=0GsVJsS6474 if you can't do a "full plank pushup" they have a good pushup "ramp up" method.

1 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6563593/

2 https://pubmed.ncbi.nlm.nih.gov/27680663/

3 https://pubmed.ncbi.nlm.nih.gov/28769100/

4 https://pubmed.ncbi.nlm.nih.gov/31023184/

5 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5446383/

6 https://pubmed.ncbi.nlm.nih.gov/26562712/

7 https://pubmed.ncbi.nlm.nih.gov/27926890/

8 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6339182/

9 https://pubmed.ncbi.nlm.nih.gov/28819746/

10 https://pubmed.ncbi.nlm.nih.gov/29800984/

11 https://pubmed.ncbi.nlm.nih.gov/30272098/

12 https://pubmed.ncbi.nlm.nih.gov/30006762/