Approach to layout of the site interface: full screen or centered with indents?

Question

Design

Dear users!
Tell me, please: why are so many portals laid out with a page indent from the screen borders?
Is it just to put ads on the background or are there some other pitfalls?
I decided to develop one project for myself, started typing a web page in full screen, got distracted and remembered that a lot of site pages are indented ... This confused me and I want to understand the reason for this.
If you can share interesting articles on this subject, it will be very cool!

Answer 1

It is more convenient to typeset when the future page width is known. If the design is "rubber", then with the current 1920 pixels. width layout is 960 pixels wide. will stretch more than 2 times, and these are long lines, problems with pictures (pictures in a column and long lines instead of paragraphs), and indeed, with all design elements. Try to use some css framework right away, for example, Bootstrap.
ps margins on the sides or from one edge - this is not for beauty, but excess width