A bit exclusive
Japanese "snack" bars are not necessarily what you would think. Just seeing the word "snack" on a shop sign is not an indication that the store serves snacks. They do serve snacks to customers, but this is hardly the original intention for going to a store like this.
It would be better to think of snack bars as, for all intents and purposes, a small bar. However these places do their business with regular local customers who build relationships with the staff members. And staff members are usually few, with maybe just a bartender, and a hostess or two who entertain the (mostly male) guests with conversation (and a lot of times the ones making and serving drinks will be the hostesses) In fact, the hostesses are likely the main reason for the return customers. Regulars to these places see the snack bar as a sort of home away from home; a place to unwind and chat before they have to return to family life.
The bar is largely kept alive by these regular heavy spending customers, so snack bars are a bit tough for a foreign tourist to just walk into, particularly if you don't speak Japanese well enough to order drinks and such.