I get confused when and why should you use specific verbs in REST?

I know basic things like:

```
Get -> for retrieval
Post -> adding new entity
PUT -> updating
Delete -> for deleting

```

These attributes are to be used as per the operation I wrote above but I don't understand why?
What will happen if inside Get method in REST I add a new entity or inside POST I update an entity? or may be inside DELETE I add an entity. I know this may be a noob question but I need to understand it. It sounds very confusing to me.