What is ad hoc testing ? What is the need for this sort of testing ?

Typically, when we talk about software testing, we typically talk about testing plans and test cases. As we get into more detail, we can get into various type of testing strategies, and explore the many testing practices that are sought to be employed during the course of testing. So, there are many testing terms such as Black Box testing, White Box testing, Automation testing, and so on.
However, there is another sort of testing called 'Ad hoc testing' that is totally contrasting to all these strategies and practices. So, what is Ad hoc testing ? Ad hoc testing is a process of testing that does not incorporate the use of test cases or any formal document that lists down software testing processes. Ad hoc testing is a form of Black Box testing, since the tester has no idea of the internals of the application, and even less idea of the API and code structure of the application.
Typically this sort of testing is done by a tester who is already experienced with the software product, and can be done in different cases - It can be done when there is a round of testing that is already done in a systematic way, or it can be done when there is not enough time to do a complete round of formal testing.
Why would we do Ad hoc testing ? Well, many teams go ahead for ad hoc testing when they have already completed their rounds of formal testing, and it has been found that when regular testers also go in for ad hoc testing, they can use their instinct to focus on areas of the application where they feel that things are a bit less secure, and find more bugs in that area. However, when a team goes in for ad hoc testing when they have not had time for regular testing, then they are getting into a high risk zone where there will be bugs in the application.
Ad hoc testing is useful also when an interim release of the software has to be handed over for demos or some other situations where it is not required that the software be of perfect quality, but embarrassing defects should not be there in the application.