Faq/General/InstallSelectedComponents/en

LibreOffice is a monolithic application, where modules like Writer, Calc, Draw, etc. run in the same process, and are actually implemented as a set of dynamically linked libraries. These libraries provide functionality that is not only used in the respective component, but also in other components: e.g., Draw implements all graphical objects (like rectangles and ellipses) that you may insert into Writer's or Calc's documents; Base components are needed in Writer when you use Mail Merge; Writer's components are essential in Base's forms, etc.

This makes it unpractical to try to limit the installed set to only subset of the components. Even if it might seem a good idea to save some space on system, it would in fact save little, and cause unexpected problems when you try to use functionality that depends on the missing module.

That is why the option to select installed modules was removed from Windows installer in LibreOffice 4.2. It still exists in Linux packages, allowing one to e.g. avoid installing Base (which is common practice in distros' packages), saving about 10 MB disk space (when the other installed files take more than 500 MB - so the saving is less than 2%) and resulting in multiple reports of problems on resources like Ask LibreOffice.