But in fact,就 has several meanings.as far as I was known,it has 5 meanings at least. 1.一……就……; Can you understand it? like "once" or"if sth happens,owing to this the corresponding thing will occur. 2.高不成低不就;This maybe difficult to you,this is like something classical expression in China. 3.就理;according to several reasons. 4.Emphasis.like this sentence “我就不去”or"我就想吃“. 5.turning point,like the English word"although'.“就算你不去学校，我也要去。” Although you don't go to school,I will go. I hope these explanations above can help you.

When used as an adverb, 就 can have many other meanings apart from 'simply' and 'just'. I don't think it makes no sense in many sentences. I guess that what makes you think 就 makes no sense in some sentences is just that it's hard to find its equivalent in English when translating such sentences. The following definitions are from 现代汉语词典. [jiù]副词。 ①表示在很短的时间以内：我～来ㄧ您略候一候，饭～好了。 ②表示事情发生得早或结束得早：他十五岁～参加革命了ㄧ大风早晨～住了。 ③表示前后事情紧接着：想起来～说ㄧ卸下了行李，我们～到车间去了。 ④表示在某种条件或情况下自然怎么样（前面常用‘只要、要是、既然’等或者含有这类意思）：只要用功，～能学好ㄧ他要是不来，我～去找他ㄧ谁愿意去，谁～去。 ⑤表示对比起来数目大，次数多，能力强等：你们两个小组一共才十个人，我们一个小组～十个人 ㄧ他三天才来一次，你一天～来三次ㄧ这块大石头两个人抬都没抬起来，他一个人～把它背走了。 ⑥放在两个相同的成分之间，表示容忍：大点儿～大点儿吧，买下算了。 ⑦表示原来或早已是这样：街道本来～不宽，每逢集市更显得拥挤了ㄧ我～知道他会来的，今天他果然来了。 ⑧仅仅；只：以前～他一个人知道，现在大家都知道了。 ⑨表示坚决：我～不信我学不会ㄧ我～做下去，看到底成不成。 ⑩表示事实正是如此：那～是他的家 ㄧ这人～是他哥哥ㄧ幼儿园～在这个胡同里。

sometimes it has no meaning or meaning right away .LIKE 我马上就来／我马上来。

Chinese language has a more flexible grammatical structure. People would add some words in daily conversations, many of which would not substantially change the meaning of the original sentence. So I would suggest learning the phrases first. You can have an idea of other meanings but don't care too much until you reach a more advanced level. I will explain it with examples. (1)when used in phrases When 就 is used in a phrase, it usually works with one or two other words. Examples of these phrases include those mentioned above, 只要……就，一……就. The first one means "as long as", and the latter means "once" or "immediately after". examples： 只要经常练习，你就会进步很快。You will make progress as long as you keep practice. 他一下班就去了酒吧。He went to a pub immediately after he finished his work. (2)when 就 is used alone in a sentence It usually adds some details to the sentence but won't substantially change the meaning. These details often are feelings, emphasis, etc. For example, 他用的就是这本书. It literally means this is the very book that he uses. When 就 is taken out, 他用的是这本书, it means this is the book he uses. This is an example of emphasis. 他十五岁就参加工作了。 He started to work when he was just/only 15 years old. Again, when 就 is taken out, 他十五岁参加工作, it means he started to work when he was 15 years old. So again, it is not a big difference. But if you look up it in the dictionary, e.g. in xiaokaoy's comment, you will find out that they fall into two different explanations. I don't think there's a need for that. Don't be frustrated , just stick to the basic meaning of the word.

Actually this character has many usages. One that I learned is the structure 一... 就.... meaning "as soon as..." E.g. tiān yí liànɡ, wǒmen jiù chūfā le. 天 一 亮， 我们 就 出发了。 As soon as the day became bright, we set off. yì huíjiā, tā jiù xiǎnɡ 一 回家， 他 就 想睡觉。 As soon as he gets home, he wants to sleep. wǒ yì zhīdào xiāoxi, jiù ɡàosù nǐ. 我 一 知道消息， 就 告诉你。 As soon as I know the news, I will tell you. Hey, by the way, if you want to find an online teacher to improve Chinese, I will recommend http://www.ichineselearning.com/ Cheers!