Page with is a view to see the entire page, while optimal view is a view to see
the entire margins. LibreOffice shows documents in print layout view and in
this view, the entire page is visible including its borders. This is the normal
view for most word processors (MS Word, Google Docs, Calligra Words) and there
isnt a need to change it in LO.
You can switch to Web Layout ( View > Web Layout ) if you want a view that is
without the grey areas around the page.