Sure, the chance to straighten out the claims later is a big part of provisionals. In fact provisionals don't even need claims, but for certain international patents to get the filing date you need to include at least one claim to make it a "patent filing" for those countries. Provisional patents are a US specific thing.
The way to think of a provisional is that it's a way to get a description on file by a certain date. Then if you subsequently file a patent application within that time you can refer to the provisional to get the benefit of the earlier filing date. The claims in the actual patent are the ones that matter and they refer to the description in the actual patent. In fact I'm not sure the examiner really looks at the provisional at all, since the claims and description of the patent application itself are what matter in regards to granting the patent.
So what does the provisional do? Example: let's assume bicycle cranks didn't exist and I filed a provisional 1 Feb 2013 for a crank with an odd number of teeth. I just wrote it up in relatively casual style ("a wheel with an odd number of protrusions around its circumference sized such that links of a chain could fit between them, and pedals attached to radial posts").
Then on the 15th of January 2014 I file a patent application for a bicycle crank with odd and even numbers of teeth. My description now takes up several pages and includes drawings done in the crazy style required by the patent office. My provisional just included a hand drawing or two and a couple of sentences of description.
If I tried to sue you over this patent for making bike cranks with an even number of teeth, and you had begun in July 2013, I'd be out of luck. But if you used an odds number of teeth my protection for my granted patent would extend back to Feb 2013.
I could still lose because the casual description might be ambiguous enough that you could convince the jury that it didn't really describe the actual, patented invention. Because of this patent attorneys try to make provisionals as close to real patents as possible, and often urge you to simply skip the provisional step.
(BTW there are domains where patents make sense. Where most NH readers are, in software, I consider patents an abomination).
