The disadvantages of not knowing how to keep and gain audience attention can include:

• The audience will get bored and start to block out your presentation as a whole
• people will become disinterested and may get up and leave in certain situations
• It could be due to the fact that the speakers presentation was not very well prepared and planned out
• Sooner or later, throughout the presentation, it may begin to seem like a waste of time
• Because your audience is not paying attention to what the speaker's topic is about, there is the possibility that they may miss you on key information that the speaker may have to offer.

There are some advantages of knowing how to keep and gain audience attention:

• your audience will be come and stay interested in you and your topic
• knowing that they are listening to what you have to say, they are receiving key points of information that they'll remember later on
• gives the speaker the ability to get their message across to their audience more effectively 
•  Lets the speaker know that they prepared an effective and strategic presentation