Organization tends to save time, especially in complicated questions. If you have, say, an average speed problem, you have 9 numbers to keep track of (the distance, time and speed for each of the two parts of the trip, along with the total distance, time, and average speed for the two parts combined). If you write those numbers chaotically on your page, you can lose a lot of time trying to find the number you need to carry out the next step in the solution. If instead the numbers are neatly arranged in a table, you never waste any time trying to find the next number you need to use. You need to write the numbers down no matter what you do, so it's vastly better to write them down in some organized way.

Now, I don't like tables much, and I tend only to use them for average speed problems. But in, say, overlapping sets problems, I'll almost always draw a Venn diagram, and in a mixtures or weighted average problem, I'll very often draw the averages on a number line. It can be especially helpful on GMAT DS questions to have ways to usefully visualize information, since you can often then 'see' just what information is there and what's missing. That's a lot harder to do if you're staring at a lot of letters in an equation.

Of course, on easier questions it won't matter much what you do, but on harder questions, I find these kinds of methods indispensable.