First, you would have to look at the time I got Google Voice. It was 2005, and unlimited voice plans were prohibitively expensive. At the time, I had a home phone, and a cellular phone. Some people that had both would call my cellular phone (therefore using minutes) to ask me if I was home (even though they had the home phone).
What I did was get a GrandCentral number which Google eventually bought. The number works much as it does today. At the time, I had unknown callers forwarded to my home phone, and known callers forwarded to both the home and cellular phone. I then changed my cellular phone number which forced everyone to call the GrandCentral number. Obviously, if I was home, I would answer the home phone.Eventually, UMA (predecessor of WiFi Calling) became available, but I still kept the system setup where people will call the Google Voice number to get me.
Since I had this same phone number since 2005, and Google does not charge for Google Voice, I keep the number with Google. The rationale is simple, if you known me since 2005, then you would know my Google Voice number, and therefore – can reach me. However, if you lost my number, a simple Google search for my name will yield one of my pages in the first page of results.
Even though I have unlimited calling, I don’t want to give my cellular phone number to everyone in the world. Therefore, the Google Voice number remains as a forwarding number, and is public to allow anyone to reach me. Calls are directed as based on my Calling Rules. Those I am OK with giving my cellular number will hopefully have the respect to give my Google Voice for those that wish to call me.