They do need to get out of the water, it benefits them in a couple ways.
1. they eventually drown
2. Breathing air is good for their insides
3. Being able to get dry i'm sure is comfortable and its beneficial to the shell
4. The resistance of gravity makes the crab much stronger, as while they are in water they can move much more easily, they are much "heavier" outside.
And i'm not sure how full saltwater affects them, but the land is a must have. They actually prefer to have alot of land , and if you have multiple crabs you need alot more space or they will go to war. Crabs are territorial, claim their turf and with a couple hours of observation with most crabs you can see how the tank's territory is divided. You also need many hiding places for the crabs.