By pressing one of this buttons you can insert the brackets. First button is for table brackets, second for a row and third for a column. The GUI is really basic and so is the functionality of tables. I've seen them being used very rarely, most users prefer to just manage the spacing and formatting to create something appearing like a table. I can understand why one would want to avoid using tables in bitcointalk. Their functionality is really limited, they're hard to figure out and the formatting they're given makes them appear odd.
Most important thing about the design is that there are no lines between the rows or columns. Adding ascii characters in an attempts to make artificial borders for the tabled text can break up the design. Headlines are also disabled for some reason.
SMF has some decent capabilities and I can't see why they aren't put in use when it comes to tables. For such a basic functionality, the new forum software could might as well benefit from using markup formatting.