CSS Frameworks are not for beginners. They're geared at developers who need to hit the ground running on a website.

I'd actually say to stay away from them until you've been doing front-end for awhile as the non-semantic naming conventions are not something beginners should be using.

I'm sure you could adjust positions with specificity, but in my opinion, that goes against a Grid Frameworks purpose.

The grid adds a base layout which you would then customize by adding classes and ids on top of the default grid classes. You can definitely adjust positions with pixel accuracy. The grid is useful because it helps with responsive web design: everything resizes and repositions when you change your browser size or use a mobile device to view the webpage.