The answer is very simple: they don't want to pay their prospective employees any more than they have to. If they put a job listing for $70k and a developer currently making $50k applies and gets it, he gets a 20 thousand dollar raise.
On the other hand, if the employer requires the applicant's current salary on the application, and Mr. Qualified Engineer applies, they can offer him $55k instead (just enough to make him consider the offer if he's unhappy with his current job).
Using this strategy, the employer can make sure their employees never know their real worth, since every raise/promotion/new job they get is based on a "just good enough" improvement over their current salary.
On the other hand, if the employer requires the applicant's current salary on the application, and Mr. Qualified Engineer applies, they can offer him $55k instead (just enough to make him consider the offer if he's unhappy with his current job).
Using this strategy, the employer can make sure their employees never know their real worth, since every raise/promotion/new job they get is based on a "just good enough" improvement over their current salary.