The ruler as it's currently implemented in LibO is only useful for looking up the current formatting, not for formatting as such. Moving the markers on the ruler with the mouse is highly imprecise and does not allow to set precise formatting (such as, e.g., 2cm exactly). While Styles may be used to work around this problem (and that is encouraged, as far as I can tell), the ruler is still much less useful than it might be and than it is in many other similar office applications (e.g. AbiWord), which steepens the learning curve for people migrating from other applications and complicates quick formatting of individual sentences and paragraphs.
Steps to reproduce:
1. Type some sentences.
2. Try to format them by moving the ruler markers with your mouse. Try to get the ruler to align exactly to 1.50 cm or 2.00 cm, not to 1.51 or 1.49 or 2.02.
3. Give up.
The ruler markers are only useful for telling the current formatting. Using them for some quick formatting, as in other applications, is nearly impossible.
Currently LibreOffice functions as it was originally designed, so it's not a bug but a feature extension request. In many other office applications, including AbiWord and Microsoft Office, it's possible to snap to ruler increments and that makes the ruler useful for formatting. In LibreOffice, the typical usage scenario is to use styles-oriented formatting and potentially to hide the ruler altogether because it can't be used precisely anyway.
Platform (if different from the browser):
Browser: Mozilla/5.0 (Windows NT 6.0; rv:11.0) Gecko/20100101 Firefox/11.0
*** This bug has been marked as a duplicate of bug 38144 ***