One of the important features of the web is that visitors and adjust their “view” to be independent of the device, screen size and/or other preferences and constraints. So pages can be viewed with super-size font by visual impairment or smushed down to view on a little netbook screen, for example.
By setting font size relatively, it doesn’t matter how big the font or window are, the browser has the flexibility to provide a good viewing experience.
The pre tag is most frequently used to include programming code on a web page. This is at “traditional” rendering of code. This helps visually with indentation, for example, that helps with code readability.
Most fonts are proportional – an “i” is much narrower than a “w” for example. Sometimes this is good – it helps your eye travel smoothly as you are reading. Sometimes it is important to limit or control the number of letters or information on a line. With proportional font, these can vary a lot.