they keep bitcoin in their wallet for long periods of time because they expect that bitcoin price in the future, it can multiply many times the current price
Well this is certain because bitcoin now is having a lot of users because of the prediction that it will be having a price a lot more than the current price. A lot of people use bitcoin, so there is a lot of development that needs to be done and I think the more the demand, the more the price, the more it is good to save bitcoin for the future.
It is the best to save bitcoins for future now because now they are affordable but once they touched 5k dollars, it wont be easy to garb bitcoins. There is no doubt that bitcoin market is growing damn fast day by day and there are maximum chances of bitcoins becoming the most expensive assets of the future.