Keeping my cell number private for work related calls, being able to text from the computer, saving different voicemails, checking call logs, easily blocking numbers, being able to save and forward voice messages, visual voicemail transcription
I would absolutely reserve a Google voice number. It is useful for more than just business. Google Voice is great as a "disposable" phone number for telemarketers, etc. I use it for all my savings cards, like grocery rewards cards. I give it to websites needing a phone number. It allows me to keep my number safe. It also allows me to keep my personal cell phone number private. I frequently use my cell to speak with students and love the fact that I can keep my personal number private.
For work, the ability to see my own call logs easily, block numbers, forward voice messages, save and change voicemail, use Do Not Disturb, star messages, see messages transcribed with visual voicemail, easily link or unlink a phone, set up voicemails specific to certain callers, and text from the computer are all things I couldn't live without in my line of work.