What you see above is the back pocket of a pair of cotton trousers that I regularly wear. Recently the button at the back pocket went missing. I believe it happened during ingress of a car I was driving at the time. When sliding in the back pocket, and the button, would rub against the seat back and cause it to detach itself.
Actually buttons on jackets, shirts and trousers would somehow end up missing sometimes. Even if the clothing may be hand sewn, a designer piece or from a factory outlet store the chance of losing a button is always there. So it actually pays for a person who is into how decently dressed he or she is to pick up the simple skill of re-attaching a button to one's clothing.
The best thing about doing this yourself is that you can add a little flair to the item of clothing. The trousers in question came with a normal style of machine stitching. Now I've used the Crows Foot stitch for the button it is also called “fleur de lis” by some. The thread is sewn from the first hole to the second hole in a straight line. From the first hole, the thread is then sewn diagonally to both sides. The crows foot is one way you can tell whether a button has been sewn on by hand or by machine. It adds a little more flair to the clothing I end up repairing.
But heck, you'd have to be really close to tell. And if I was wearing this pair of trousers it would mean someone checking out my behind. Nope. It is definitely for personal satisfaction that I do this.