Combine Columns is used to merge two columns into one.
If the CSV table is
Combining the columns 1 and 2 results in
To use Combine Columns, select the columns that you want to combine and go to CSV > Columns > Combine Columns….
The Use first non-empty value option will combine columns by taking the left-most non-empty value for each row. Here is a case where this was useful.
Split Columns is the inverse command which splits a column with the given separator. It is under CSV > Columns > Split Columns….