Advantages
Remember that you are generally in control of the content you are presenting, so you can include topics that you want to talk about. Additionally, you can use personal examples from experience to support what you are saying. Since you are an authority on the topic, you want to speak with conviction like you really mean it. Your delivery will naturally be more conversational and spontaneous. Since you are not prepared with pages of notes, you are more likely to speak directly to the audience just like if you were speaking to another person in a conversation.
Disadvantages
Since you are not well-prepared, you may have difficulty thinking of what to say or formulating the ideas once you get up to speak. Although you are familiar with the topic, your speech may lack details and supporting information. If the audience is passive and does not ask questions to guide you, you may overlook some significant content. Hopefully, someone in the audience will ask questions so you can fill in gaps. Additionally, impromptu speaking is rarely appropriate for occasions which require more reasoned discourse with supporting ideas or more formal events.
Source: Boundless. “Impromptu Speeches.” Boundless Communications. Boundless, 25 Jul. 2016. Retrieved 02 Oct. 2016 from https://www.boundless.com/communications/textbooks/boundless-communications-textbook/delivering-the-speech-12/methods-of-delivery-62/impromptu-speeches-248-1482/