Why long neck bottles 

The long narrow neck is easy to use as a grip, wider handles are more difficult to grip continuously for long periods of time.Sometimes the anti-slipping is emphasized by a bulging neck which prevent slippage further like this. in this case due to the bulge you can grip the bottle with just your thumb and forefinger.

Purpose of an extra holes in a sneaker

Those extra holes can actually serve an important purpose. They can help make shoes fit better, and thus prevent blisters.

Small random buttons on jeans

The purpose of the buttons, which aren’t really buttons at all. They’re called rivets, and their purpose is to prevent the the denim from wearing out and ripping at the seam. 

Secret underwear pocket

protect against friction from synthetic materials which don't protect against rubbing, bacteria and sweat. 

Padlock hole

As padlocks are most often used outside, the holes at the bottom prevent it from rusting and letting the water runoff.

A loop on the back of a shirt

Legend has it that these loops were designed for East Coast sailors to hang their clothes on a wire line to dry instead of hangers. What’s more, these loops saved the shirt from getting wrinkles and the sailors could wear the shirt again the next day!