The main driver for this is safety for his workers (along with other possible motivations). There was a hit and run incident that injured 3 employees. So this tunnel will mean safer commute to work.
Not sure if he is totally serious about 'The Boring Company' but I could see him doing it for sure if he can get this first tunnel done without major incident. This will have a use possibly in the Mars colonisation plan so as he has shown before he will take a gamble at creating another company if he feels it will benefit that plan in terms of technology and also increased revenue.