Their are two menu locations:
Primary - The main navigation
Secondary - The smaller navigation adjacent to the search bar
The secondary menu is simply hidden for screens smaller than 768px and the full primary menu is displayed. The drawbacks are obvious; potentially important content in the secondary navigation isn't available . A dedicated 'handheld' or 'mobile' location will allow folks to create a menu specific to mobile with complete control of its contents...but will increase menus n submenus if you are having menus according to category. Looking forward if I can display secondry menu too along with primary handheld menu in storefront theme.woocommerce.