The use case for the tool itself is great. I have been using online services before to extract individual frames from a sprite sheet. It's also nice that there is an option for image filters like greyscale.
Some suggestions for improvement: I have only see the option to adjust grid size settings with mouse drag and drop. It would be great if there were the following options to have a more accurate/faster way of getting the right grid size:
1. textfield inputs for defining the x and y cell count of the sprite sheet. for example 4 on x and 6 on y would split the texture into 4 columns and 6 rows.
2. textfield to define the pixel size for an individual cell.
3. Optionally include a textfield to define pixel padding. each sprite cell will have this amount of pixels removed from its borders. I don't think it's necessary but I have seen that some sheets are set up for some padding too. Usually uncommon though.