Many enjoy a beautifully tiled floor or wall, and ceramic tile is very durable as a building material. Due to being waterproof, it is ideal for placement in bathrooms, kitchens and laundry rooms. The problem is that the grout in between the tiles will degrade over time, and thus no longer seal the area well.
If you are able to spot cracks in the grout when using a magnifying glass, the grout needs to be replaced.
You don’t need to replace all the grout at once, but you should try to match the color of the existing grout as well as possible to improve appearance. It will look better if you decide on an evenly shaped area to redo, even if the grout might not be damaged in the whole area.
You don’t want to cut the old grout away too deep. Removing about half the grout should be enough to make room for the new grout. After the old grout is removed, you want to clean the area very well before applying the new grout.