When the grout on your tile wall or floor starts to crumble and crack, you should extract the grout and re-grout the area, so moisture doesn't seep into the underlayment behind the tiles and cause bigger problems. Getting the old grout out is the hardest part. If there's a small amount of it, you can use a hand grout saw, but for bigger areas, a better solution is a grout-extraction disk, which can fit onto your angle grinder.
Clean the tile surface, including the grout lines, with household cleanser and a sponge, so the grout-extraction process doesn't grind dirt into the edges of the tiles.
Set up your angle grinder with its grout-extraction disk.
Hold the angle grinder so the disk is sitting on the grout line, at one end, facing in the direction of the line.
Turn on the angle grinder. Gripping the tool with both hands, dig down into the grout while pushing the disk forward along the line. Keep the disk sunk about 3/8 inch down into the grout, so it's at the bottom of the grout but not digging into the material behind it.
Continue moving the disk forward, digging out the grout and taking care not to damage the surrounding tile. Do the whole line, end to end.
Vacuum up the pulverised grout from inside and around the line. Repeat for each line of grout. The lines are now ready for re-grouting.
Wear eye protection while extracting the grout.