* They are one of your only chances to get people interested in actually reading your paper.
* When people are reviewing your CV many will just look at the title of your papers, as well as the journal they are published in. Interesting, diverse, informative, and understandable titles create a good impression. Boring, repetitive, and highly technical titles create a bad impression. Make sure all your papers don't have essentially the same title!
* They are fun.
What I generally do is to write down as many as five possible titles for the paper and then consider their relative merits and discuss them with co-authors and colleagues. This helps sharpen the title.
If you want to see some good examples look over the publication list of Roald Hoffmann. Here are just a few from the past decade:
A Molecular Perspective on Lithium-Ammonia Solutions
A Little Bit of Lithium Does a Lot for Hydrogen
A Bonding Quandary—A Demonstration of the Fact That Scientists Are Not Born With Logic
The Contributions of Through-Bond Interactions to the Singlet-Triplet Energy Difference in 1,3-Dehydrobenzene
“Half Bonds” in an Unusual Coordinated (S4)2- Rectangle
Emergent reduction of electronic state dimensionality in dense ordered Be-Li alloys
A Quantum Mechanically Guided View of Mg44Rh7
The Many Ways to Have a Quintuple Bond
A Pnictogen of Peculiar PostureOn the other hand you could go the Ph.D comics route: