Yes, as an example Atlanta put Andrew Jones on waiver this month and the Rangers claimed him. Lets assume Atlanta tried to trade Jones to the Rangers in July but couldn't get anything worked out. By Jones being claimed by Texas Atlanta has 3 choices 1) Work a trade to the Rangers, 2) Simply let the Rangers have him (Which the Braves would never do) or 3) Pull Jones back off Waivers. Should #3 happen then the Braves are stuck with him for the rest of the season.
Now this is where it gets fun, let's say the Red Sox and Yanks (A-Rod haters feel free to snicker
here) need a third baseman in the worst way and the Rangers waive Hank Blalock. The Blue Jays, seeing whats going on, puts a claim on him. The Blue Jays have no need for Blalock, all this claim does is PREVENTS their division rivals from grabbing Blalock. This happens all the time (Forgot which GM it was but he bragged that he would claim on average 25 guys a year just to make sure his rivals don't get them). This can backfire on you. Burn your rivals enough times and should the situation be the opposite they won't hesitate to burn you.
Like the example above with Braves/Rangers and Jones. Once claimed the Braves have 48 hours to do one of the three above. Now they know the Rangers really want Jones so they'll call Jon Daniels (Texas' GM) and tell him "Allright Jon, you want Jones - you have 48 hours to make an decent offer". The Braves hold all the power and can do what they want, it would up to the Rangers to appease them to the point where they would trade the player, since Jones is a good player they won't simply let him go. Now if Jones sucked Atlanta wouldn't hesitate in letting the Rangers have him.
Good question, don't know why they do this. In fact I think the trading deadline should be at the end of August as July 31st is too soon.