Dogs eat grass for various reasons. For a start, it contains roughage, so the fibre can help digestion and bowel movements. But since you’re reading a blog about dog welfare, I’m guessing your dog is beautifully cared for and doesn’t have poor nutrition. If it’s a possibility (illness, malabsorption, etc) please talk to your vet.
Some dogs just like the taste of fresh grass. It seems a weird choice, but then they will also eat liver and sardines, and at the same time! (Personally, given the choice of only those 3, I’d eat the grass.) It won’t do them any harm but obviously don’t let them eat excessive amounts in case of an intestinal blockage, or grass that’s been sprayed with pesticides.
Dogs may eat grass because they’re bored. Is your dog getting regular, sufficient exercise? Regular, sufficient, social engagement with you, other dogs and/or people? If so, the main causes of boredom are solved, and this is an easy one. When your dog’s home alone, leave access to a window, or garden gate, to watch the world go by. Plus a couple of dog toys for self-entertainment. Or some hidden little tasty treats that need to be sniffed out.
Most dog people have seen an obviously uncomfortable dog with a grumbly stomach, rushing out the door to eat some grass. Grass lowers the pH in the gut. A dog will eat grass to try to soothe an upset tum. If your dog then vomits, it was probably going to happen anyway. Obviously, but it still needs to be said, monitor your dog afterwards and if he/she is unwell, speak to your vet.