Grids gives order to layouts and allows the eyes to flow easily across the orderly content. Grid layouts are suitable for information type of content, e.g. websites, newspapers, magazines, editorial, annual reports, statistics, etc.
When I design a website in PS, I use guides for the grid. Here're 2 examples of my grids on a working layout.
It's not for everything. It's just another way of giving composition to a visual layout.