This is why you are seeing more horseshoe crabs stranded on the beach than in previous years. Most do not wash up on the beach, but come ashore to spawn and the waves turn them over stranding them on the beach making them vulnerable to predators. (shore birds, raccoons etc.)

Horseshoe crabs lay their eggs between the high and low tide marks. This helps to keep the eggs moist until they hatch. The shore birds you see feasting on the beach are most likely eating the eggs although shorebirds will sometimes be seen feasting on the overturned horseshoe crabs too.

Turning over horseshoe crabs and helping them back into the water, while in the big picture won’t make much of a difference, to the individual horseshoe crab it could mean the difference between life and death. While some of them would probably be able right themselves and get back to the water when the tide comes in, many could die from dehydration and predators before that happens. If it makes you feel better, by all means help them. I always try to help an animal in distress as long as it’s safe and feasible.