Coming straight to the Questions. Let’s explore the reasons as in -
Why do we have a tiny pocket in our jeans?
Most of us wear a jeans, many of us have a habit of using the small pockets which is literally a pocket inside the main pocket. But the small pocket is Not for ‘chillers’ (change) it is not for the key either. The real reason why we have the small pocket in our jeans is actually due to this reason : – The reason goes back to to centuries when most people used chain watches. Cowboys used to wear their watches on chains and keep them inside their waistcoats. However, the problem was to segregate the watch from main pocket and therefore this small pocket was introduced. The pocket was introduced by Levi Strauss in a bid to be more efficient and safe and still they are, even after more than 200 years of augmentation. They are basically watch-pocket. Info Source - Quora
Why do we have small buttons on jeans pocket?
The reason again takes us back in history. The reason why we have small buttons on is : Earlier, Jeans were worn by the working class population that was subjected to hard physical labour. The work demanded a lot of moment and labour which concludes in over-stress on jean’s pocket area. This was a problem for them because they needed those pockets. To the rescue, came a tailor named Jacob Davis, a local customer of Levi Strauss & Co., who came up with an ingenious idea in 1873, that would ultimately be hailed as genius. Jacob thought of placing metal copper rivets as fasteners to reinforce the seam where the pockets attach. This held the pockets and the jeans together and prevented them from being ripped off of the jeans. 
Jacob wanted to get it patented but he didn’t have enough money! So, in 1872, he wrote to Levi Strauss and offered him a deal if Strauss would pay for the patent! info – source Image Source source
