no. Your minutes are still used.
No. Unless you put a price on google having all of your call/sms data.
It's a web-based pbx. You can forward calls from your gv number to another number to protect your "private" number. You can block certain callers from calling you based on predefined rules. You can send sms messages through gv over your data plan instead of your texting plan (for example, i don't have a texting plan anymore because i barely use data). These are just a few examples of how gv can work for you.