I am a beginner in HTML and haven't started learning CSS yet. Is there any reason where one would prefer to use
They are nothing like each other other than sharing the first letter :-)
From those links:
A paragraph is typically a run of phrasing content that forms a block of text with one or more sentences that discuss a particular topic, as in typography, but can also be used for more general thematic grouping. For instance, an address is also a paragraph, as is a part of a form, a byline, or a stanza in a poem.
preelement represents a block of preformatted text, in which structure is represented by typographic conventions rather than by elements.