Often, I would like to have two different views of the pdf file associated to a tex file.
A typical use case is: you wrote a mathematical paper including several theorems, and the proofs are in an appendix at the end of the paper. To check that the proofs are correct, you often need to go back and forth between the main text containing the theorem statements and the appendix containing the proofs.
A possibility would be to open an external pdf viewer, but this "locks" the file and prevents TeXstudio from compiling and updating the pdf file.
The ideal solution would be to have a three-subwindow split view in TeXstudio, with the source tex file in one subwindow and the pdf file in the other two, so you could see different pages in the two views of the pdf file.