A patent application is a fairly complex document, and the value of the corresponding patent is critically dependent on the most cryptic part of the application, the claims.  Claims can be "broad," meaning that they cover a lot of simple variations of an invention, or "narrow" -- they only cover a few variations, so the patent is easy to get around.  For example, if the invention is a widget with two parts that are screwed together, and the claim says "...part A is connected to part B with a screw..." then somebody could copy the widget, as long as they connected the parts with a rivet, nail, glue, or some other type of fastener*.  On the other hand, if the claim doesn't specify how the parts are connected at all, then the claim might be so broad that it's unpatentable in view of a prior-art widget that had the two parts formed as one monolithic piece.

Drafting precise, yet broad, patent claims is one of the hardest things an IP lawyer is called on to do.  Almost anybody can write a claim, but to write one that a patent examiner will accept as patentable, and then an infringer will understand and respect, is quite a feat.

An experienced IP lawyer will also have a good feel for the direction a patent prosecution is likely to take, and write the specification with good fallback positions in mind.  The initially-filed claims should often be aggressively broad (if the original claims are allowed in the first Office Action, it's an indication that an even broader patent could have been sought).  Ideally, one's original independent claims will be rejected, but many of the broader dependent claims will be found allowable.  However, if all the claims are rejected, it's important to have enough detail in the specification to support narrowing amendments.  Without such detail, one's options are severely limited.  Of course, it's also important that the details be correct!  Claims that recite impractical or unworkable combinations are worse than useless!

Finally, there are a number of simple mistakes a non-lawyer might make in drafting a specification, that can make the patent vulnerable to attack and therefore less valuable.  A competent IP lawyer will avoid these mistakes.

*This is an oversimplification -- here, the patent holder could argue that a rivet or nail was equivalent to a screw in this case, and so the copy of the invention would still infringe the claims.  Figuring out what claims mean and exactly what they cover often involves some advanced legal, as well as technical, analysis.  But the point is, a slight change of words in a claim can make an enormous difference in what the claim covers, and therefore in how much the claim is worth.