I agree with @kunalbhat that this might not be the best area to ask this but since you did I will try to answer it.
The grids are designed allow for speed and adaptability. Speed in multiple senses. The first part of the speed is the speed of writing the code. You can easily get the layout you want when you are using the grid system and everything aligns correctly. You don't have to remember your tables and columns and col-spans, etc.
The second speed is modifying your code. Inevitably you will need to go back and make changes, with a grid this is easily to do. Changing a col-md-7 to a col-md-6 easily makes a little tweak in the layout of your page that can easily be tracked and performed.
You mentioned responsive design, both Bootstrap and Foundation have responsive grids. The grids will snap to different sizes based on the viewport size. However you have control as well. For example if you want something to take 1/12th the screen in desktop, 1/4th on a tablet, and 100% on a phone that is easily done with Bootstrap and Foundation, both have grid classes that target specific viewports.
The also provide visibility classes based on those viewports.
For the "bloated" part that is easily solved. Using SCSS you can easily only import portions of a library. For example for one project I was on I only imported the Grid and it was considerably smaller.