Whatever you call it—landing board, alighting board, or front porch—bees do not need one. Hollow trees, attics, and eaves hardly ever have landing boards and neither do swarm traps.
In fact, if your landing board is constantly wet or covered with snow, it can be a definite negative. Bees can get their wings “stuck” in water and be unable to right themselves. Snow accumulation on the porch can cover up the opening, preventing egress from the hive and reducing air circulation. Landing boards also provide a convenient platform for some predators such as mice.
On the other hand, bees will use landing boards to hang out in hot weather and, curiously, to land. If they have one, they will almost always use it.
If you like to watch your bees come and go, a landing board is a great thing. It gives you, the observer, a bit more time to see the bees before they enter the hive. They land, walk to the entrance, then quickly disappear. But during those few steps, you get to see the color and size of the pollen loads, and the distended abdomens typical of nectar-carriers. You can see if they are young or old, if they are male or female. Sometimes you see a queen returning from a mating flight.
Interesting things come from the hive as well: rejected pupae, bees with deformed wings, malformed bees, injured bees, diseased bees. Without a landing board these things often drop straight to the ground and you’ve lost a good opportunity to learn about your colony’s health and activities.
So while they are by no means necessary, I think landing boards are a plus—especially for the beekeeper who is mesmerized by colony life.