Welding workbenches are a basic necessity for any welder or metal fabricator to have, since wood is flammable and therefore would create a very real fire risk; rendering wooden tables useless for welding projects.
Welding workbenches are generally made of sturdy steel, which eliminates the fire risk as well as providing a tough, durable workspace. Its main purpose is to provide a welder with a stable surface to work on. Most welding workbenches also have adjustable legs, meaning the welder can adjust the height so they can sit or stand ergonomically while working, which can make all the difference if you weld for hours at a time.
The surface of the table will usually be coated in a zinc coating to help prevent rust and hot particles from getting stuck onto the table, while still allowing it to conduct electricity. Some tables also come with holes or hooks to help clear the workspace of cables and provide a safe place to set down the welding gun.
Some welding workbenches are more advanced than others, for instance some may come with fume extraction exhausts to make your workspace that much safer.
Deciding on the size of your welding table should be determined by how big your welding projects are. Obviously the bigger the piece you're working on, the larger the table you'll need. If you use your workshop for multiple projects and applications, it might be best to purchase a portable/foldable welding workbench, which will allow you to free up some space if required. These are best for supporting lighter loads. Otherwise, there are workbenches with fixed legs, that aren't portable but allow you to place much heavier items on their tabletop.
Of course, many tend to make their own welding workbenches to suit their exact needs, so make sure if you're not buying from a brand you know (such as Aardwolf, Farmtec and Hafco), that you inspect the table and ensure it is safe to use, as well as that it meets your needs.