Search finds a product by name across every enabled category, so you can add it to the basket without browsing the category tabs.
Steps
Tap the magnifier at the top right of the menu.
Type part of the product name in Enter Your Search Here.
Tap the product in the results.
Tap Done on the product panel.
Tap Cancel to go back to the category tabs.
What you should see
The product goes into the basket. Cancel brings the category tabs back.