One way to consider this is that

h1 is the largest possible heading front

h2 is the next size smaller and so on

Usually the most common or base font is a p tag  <p> </p>

but there are smaller fonts like in footnotes

p tags are mostly around 11pt, 12pt, 13pt size fonts

h tags are usually larger and have some bold applied

but someone would make a p tag adjusted so that it is bold of any size

Why then use h tags if p tags can do whatever an h tag does?

Because when the computer is told it is a p tag the computer will not regard it as a heading but only as regular paragraphing.

Once it is an h tag no matter what size it is, the computer will regard that as a heading which is important across the internet as an alert to search engines.