Money (initially). Contracts were with companies wanting software professionals with an engineering background.
Cons
Each year I found my initial skills, training, and education used less and less. As a result, I found my career more in jeopardy. This hurts morale tremendously, especially during poor economic times when jobs are scarce and contract jobs are non-existent.
I've worked with Donatech off and on for 12 years. I only have to leave because I hit Rockwell's limit for a contractor. Then a year later or so, Donatech gets me back in. I love working for these folks.
Cons
Their clients are in locations that I don't like to visit.