One of the things I didn't foresee about becoming a professional writer is the impact this has on my own reading. I have much less reading time, for a start. I also find it very difficult to switch off and read fantasy fiction purely for enjoyment while I'm actively working on a book. I'm thinking so critically about my own work that I cannot escape that mindset. So I save up the fantasy fiction that I really want to escape into for my holidays now and mostly read crime and mystery fiction for relaxation.
Mind you, I find my tolerance for any kind of fiction that's poorly written, under-plotted, inadequately characterised etc, has pretty much evaporated. I don't have reading time to waste now, where I used to always finish a book I'd started, no matter how unsatisfactory I found it.
Reviewing is a way I can still read fantasy and SF while I'm engaged in writing my own books. I can apply that critical mindset in a positive way. It helps me keep current with the state of the genre as well.
On a personal level, the discipline of concentrating on things like characterisation, plotting and the most effective use of description and dialogue means I go back to my own work with a clearer perspective. I can't allow myself sloppy work if I'm criticising it in others.
Essentially though, I just like recommending books that I've enjoyed to other readers. I've been doing it ever since I started reading. It's what book-lovers do.
So far, I've reviewed for the webzines The Alien Online Emerald City SFRevu and the print magazine Interzone.