A welding table turns improvisation into a repeatable process. You set parts on holes rather than by eye, so the second piece comes out like the first. On series production that is often a bigger gain than a new power source, because it shortens the setup, not the welding itself.
Size the table to your largest part with 200 mm to spare on each side for clamping. A 1500 x 1000 mm top handles gates, frames and workshop structures. For large grids and machine frames take 2400 x 1200 mm or 3000 x 1500 mm. An octagonal 1200 x 1200 mm table suits work where you have to go around the part and weld from every side.
Top thickness decides flatness. An 8 mm top is enough for light structures; with high thermal stress and parts above 100 kg take 12 mm or more. Nitriding hardens the surface, limits corrosion and lets you scrape spatter off without damaging the top.
The 16 mm hole system has the widest choice of accessories and the lowest price per item. You start with a table and a few squares and clamps, then extend the set with pipe prisms, stops, bolts and clamping blocks. You will find the whole range in the Welding table accessories subcategory.