Yes, it can help you make a good deign, color, size, etc. of your site. That is why it called as style sheet.
You can easily structure the content to appear first with the rest of the source code following it. This will help outrank your competitors who may have their navigation appear first and their content last. Moreover, since you are using CSS, it reduces the file size of your html document so it is faster to load of your pages.
Re:css Well no doubt CSS is a good technology to create page semantics and layout. They make the pages beautiful. The main advantage is separate the presentation semantics from other coding.
CSS is really helpful for making a design or manage your site with different design, size etc. There is no doubt that CSS is a good technology.
Yes you can make a website using CSS. Table less site is preferred by search engine. That's way it's very demand in designing now a days.
offcourse you can use CSS for designing the website without css you wont get the real feel of colors and typography