Is it always necessary to write an abstract on top of your research work or can I immediately go over to my introduction?
This is usually specified in the guidelines of the journal, conference etc. you want to publish in. For a thesis, there will be university guidelines. I have never seen any guidelines that say that an abstract is only optional.
Ask yourself about the purpose of an abstract. A missing abstract will make it unnecessary difficult to grasp the contents of the work. So I see no proper reason to avoid writing one.