Three main reasons 

1) Your customers are demanding it! As customers start spending more and more time on smart phones, online ordering becomes a crucial way for you to be available whenever where ever your customers need you 

2) Customers tend to order more when they see all options at once in an online menu 

3) You save on operating costs significantly as most online orders are for either for to-go or delivery. You end up increasing productivity of your kitchen staff and spend less on waiting staff.