Rather than the position of the baseline being determined in relation to a text box, baseline grid determines it in relation to the page, and, when it’s on, overides any leading value applied within text boxes.
It’s a good way to get the type in adjacent columns to line up with one another.
If you set 9/10, but the grid is 14, you get 9/14. If you set 9/14.5, you get 9/28.
A good trick is to set the grid to half your leading. That way you can do stuff like applying extra between-paragraph leading with a value equal to the baseline grid.
Nick Shinn
A regular rhythm in music helps people keep track, a regular rhythm in type makes things easier for whoever’s experiencing the content. Assumptions can be made—often subconsciously—regarding where a piece of content is going to be, purely on the basis of the rhythm. Searching and disorientation are both reduced, and the speed in which content can be accessed and digested is increased. Figure 1, below, shows a comparison of a page with arbitrary values with one that has considered vertical rhythm. The difference is subtle, but the image with text aligned to a baseline grid is neater and easier to read.

Craig Grannell