I wonder, do I necessarily have to use the renewal procedure?
You don't have to, but it's not terribly user friendly to force them to click a button to connect again every 180 days.
Can I not to use it and just replace "Connected to QuickBooks" button with "Connect to QuickBooks" in 180 day?
This is perfectly fine, as long as you don't mind fielding the angry "why aren't I connected to QuickBooks anymore!?!" questions from your customers.
How is renewal beneficial from receiving a new token in 180 days?
It's much more user friendly to renew for them, rather than forcing them to click a button every 180 days.
If I should use the renewal procedure, how do I force an use to renew it within the last 30 days? I have no idea because they can just turn a blind eye to the message saying "You should renew your quick book token" which I can place in my web site.
You don't even need to tell them you're doing this.
Store the timestamp when the token was set up, set up a cron script, and whenever a token is due for renewal go ahead and renew it.