When we go about our lives (in the real world), our brains don't observe every detail in our environment at all times; instead, we focus on (or pay greater attention to) information that is relevant to the task at hand. For example, when we are driving, we are able to adjust our focal length to focus on different details, some of which are closer and others are further away, and then act on what we observe. Similarly, when we are conversing with others, we usually don't listen carefully to each and every word; we listen to only part of what is spoken and use it to infer the relationships with some of the words to figure out what the other person is saying. Often, when we are reading/listening to someone, we can use some words to infer what the person is going to say next based on what we have already read/heard.
But why do we need these attention...