I use it as my main number (not having to give out my cell phone number).
I have GV set up to only send calls to my phone during certain hours. That way, I don't get calls in the middle of the night from friends or others. People who need to get a hold of me (like for an emergency) have my actual cell number.
Voicemail transcription. Sure, it isn't always accurate, and the ones that are off are usually quite entertaining, but I can usually figure out the message without having to listen (and can listen as needed).
Voicemail messages are kept for as long as you want. Voicemail through my carrier deleted after 10 days, so I can't keep important voicemail messages without GV.
I'll load up Gmail while on my laptop. GV calls can be directed to my laptop instead of my phone. If I am out of town, calls can still get to me without international charges (when out of the country). As long as I have wifi access, I have a local US number.
I have a blocked call list on GV. Those numbers don't get forwarded to my cell number. I don't need health insurance, buy/sell a timeshare, and so on, so I can add telemarketers to the blocked list and never get bothered by those calls.
So, there are lots of things that make GV a good option.