Prior to this rule change, players would indiscriminately move the ball around in any fashion that they wished, often referred to as "bumping the ball". There were several problems with this practice. The player could inadvertently improve the lie of the area by such action; players often did not accurately gauge the proper distance to place the ball relative to the original location; and players were prone to bump the ball, change their mind, bump the ball to a different place, change their mind, and bump the ball to somewhere else before finally hitting the ball (contrary to the concept of placing the ball just once and having it in play).]
The requirement of marking the ball before lifting it helps eliminate these problems.
The second change relates to how the governing bodies treat the rule. Prior to 2004, the Appendix contained the strong admonition that the USGA and the R&A did not endorse "preferred lies" and "winter rules"; that such rules contravened the fundamental principle of playing the ball as it lies; and that the ruling bodies would ignore any requests for assistance in making rulings when "preferred lies" and "winter rules" were involved. These statements have been removed as of the 2004 rules edition. Now, it is simply treated as another local rule, no different in status than any other local rule. While this detail may seem to be a very small side note, it reflects a significant change in attitude toward a practice that used to be one golf's governing bodies thumbed their noses at.
There is one practical drawback to winter rules. Section 7 of the USGA Handicap System Manual addresses winter rules, and stipulates that rounds played under winter rules shall be posted for handicap purposes. If you keep a handicap, and play a round using winter rules, you must post that score - which will likely be lower than the score you would have shot without winter rules. You are therefore artificially lowering your handicap by using winter rules.
The choice is yours. It is not mandatory to take advantage of winter rules, or preferred lies, when the local rule is in effect. You are entitled to take advantage of the rule if it is in effect, but you are also entitled to play the ball as it lies, if you prefer playing the game in the traditional fashion.
A couple points in conclusion:
Generally speaking, winter rules or preferred lies applies only to the fairway. (It does not apply in a bunker, water hazard, nor on a putting green.) A ball on the fringe may not be moved onto the putting surface. It is a one-time placement. The ball must be marked when lifted.
Any course that puts winter rules into effect is obligated to spell out, in detail, exactly what that means at its course, so that golfers are not playing under different sets of rules;
No golfer may put winter rules into effect on his own. Winter rules must be a condition approved through a local rule;
No golfer is required to play under winter rules. If you play a course with winter rules in effect, you could choose, rather, to play it under the Rules of Golf.