Earplugs are life saviours, especially during the drab, seemingly endless metro ride or to avoid those awkward hellos when you pass by someone you don't want to make conversation with. It is also one way to effectively filter out all the peripheral chatter and the cacophony of the everyday mundane, when you need to focus. Even night-time easy listening becomes 'easier' when you have a pair of earplugs for company.
Which is why, most earphones come with either ridges or perforations on the left earphone, so that identifying which one is which, becomes easier.
The slightly upward jut of the ridges, helps identify the "L" earphone.
The linearly aligned arrangement (ridges) symbolizes the letter "L", in Braille; to aid the visually impaired.
Feeling the ridges/perforations also helps you determine which earphone is which, in the dark; so that you don't have to go through the ordeal of switching on the lights!