Today on the twits someone asked why people take the time to write reviews. Some replied that they found them extremely useful, particularly when trying to learn something about a new field/sub-field. This led to someone positing that maybe people write them as a service.
I doubt it. Reviews take a lot of time and effort. I submit that people only do it if there's something in it for them.
Personally, I think the main reason to write a review is to increase your name recognition in your field. Stake out your territory. Pee on the relevant fire hydrants.
Service to the field? Secondary at best. Maybe even tertiary after garnering the increased citations reviews tend to get.
What say you?