Sailboat companionways are an interesting doorway to land lubbers. They sometimes create some confusion, but I can assure you that every part of them serves a purpose.
The sliding hood is very important. Without it, the companionway would need to be much taller and it would be hard to fit that on a smaller sailboat. It also serves to keep water and spray out of the companionway and cabin. If a storm quickly approaches, simply sliding the hood will greatly reduce the amount of water that will ingress the cabin. It also allows good ventilation of the cabin by removing the slats and closing the hood.
The slats are the next point of contention. Doors would be much easier, and do not need to be stowed when the companionway is open, why must we have these boards? The boards serve many purposes:
They are stronger than doors
They protect against water entry to the cabin
They can be removed in stages
Doors are convenient, but if a wave hits them, they will burst open and allow the force of the wave into the cabin. Boards will help resist this force!
Doors are either open or closed, they can't be anything in between. Slats offer many options. Have all the slats in and the companionway is shut. Take the top one out and you can get air flow through the cabin while keeping rain out. Take out all the slats and it's an open door. Leave the bottom slat in and you have insurance against wave ingress into the cabin. If a wave boards your yacht and the companionway is open, all of that water is going into the cabin. Leave the bottom slat in and you have greatly reduced the amount of flooding below.
The bevels on slats help keep out water. When a wave smashes into them, they will squeeze down on each other and close the gaps, yet they can be removed with one hand. You can't do that with a door!
In time, you will find that you don't have to remove all the slats to get through the companionway. We usually only take out the top one and step over the lot of them. This makes for quick and easy entry and exists while still having the protection offered by the slats.
It may seem cumbersome at first, but with practice, companionway slats will become second nature and you'll be able to do it in the dark!
If you are in the market for a new yacht or are building one yourself, try to find one with some taper to the companionway opening. Parallel sides will cause the boards to get stuck and bind as you try to put them in or take them out. Some degree of taper will reduce this problem and make the process of inserting and removing the slats very easy. If you are already the proud owner of a yacht and you have parallel sides on your companionway, try keeping the sides of the slats lubricated with soap. This will help them to slide more easily without binding as often.
As always, practice makes perfect! Do resist the temptation to switch from slats to doors. There is a reason they have been the standard form of closure on sailboats for many years, it's because they work!