There are different possible reasons, here are a few I could think of that made me use it:
1. You want an item that starts off at a specific size and can only increase by set steps - e.g. start at width 50 and only grow by multiples of 100. Google calendar is a good example: Minimal height of a meeting is 19px then grows by increments of 21 pixel per half hour. Minimal height is slightly lower than a full multiple in order to leave some space for a margin, but steps are constant because half an hour has a constant height on the grid.
2. You can use a large grid step to effectively prevent changing the size of the element in a certain dimension. For example I could set grid=[10000,10] to prevent resizing the element on the X-axis. Yes, you can also use max- and min- width, but if you don't know the initial width then this could be a simpler way to do so. With the issue discussed here, there is a problem with dragging north which will automatically increase the width of the element by 10000 and that is a problem.
I saw the previous issue that this was meant to solve and believe there could be a solution that solves both issues at once - they do not inherently contradict each other in terms of desired behavior.
Thanks for your help!