Gates' wealth is astronomical, and every second he is richer by nearly $500. What practical use could all such money be to himself?
It is easy to quote such figures about Gates' riches, but difficult, if not impossible, to grasp what they mean in practical terms. The term "astronomical" is often used lightly in various contexts, but the similarity between Gates' wealth and astronomical distances is that both are beyond any value that a human brain can truly visualise; the bare figures can be written down but hardly convey the meaning of them.
Nevertheless some thought experiments with his wealth are interesting, and show that he could not possible "enjoy" his wealth directly.
As of early 2017 Gates is worth about $90 billion, more than the total wealth of some nations. His investments also increase dramatically in value all the time - an increase of $15 billion from 2013 to 2014 (about $40 million per day, $472 per second).
Suppose that for his 20 remaining years he spends all his money (capital of $80 billion plus income of $40 million per day, totalling $372,000,000,000) on himself - on cars at an average price of $30,000. He could buy 12.4 million of them. Assume he spends half his time with his cars (the other half sleeping, eating and visiting his optician). He would need to buy a car every 35 seconds. He would not have time to walk between them and just get in and out of the driving seats, let alone drive all of them. He could only take a tiny fraction of those cars even for short runs, and the vast majority would be duplicates of each other anyway. Alternatively, he could just buy a modest 52 cars for an hour's worth of his income and actually enjoy driving a different one every week of the year. In other words, he can only "enjoy" a tiny fraction of his money even buying relatively expensive things like cars.
Of course, he could buy more expensive things than cars (and does) such as old masters. However, there are not a third of a trillion dollar's worth of old masters in the world to be had, even if they were for sale, and at auctions he would outrun any rival bidding long before his money ran out.
Gates is in a situation like that in "Brewster's Millions", with the difference that he is allowed to give it away. The time he has available to "enjoy" all his wealth is insufficient by orders of magnitude. He has stated that he will not leave much to his children. So he will die with the vast majority of it still in the bank unless he gives most of it away before then. There is really not much else he can do with it, and the amount of money he has given away has made absolutely no difference to his personal lifestyle.
Gates has given a lot to charity, but to assess the donor we must consider that a "lot" is relative, and we are reminded of the Biblical story of the widow's mite. He may be genuinely concerned about the people he aims to benefit (anyone feels sorry for sickness victims), but he is also "buying" positive karma for himself. It is common for very wealthy men to do this towards the end of their lives - Rockefeller, Carnegie and Nobel for example. Nobel, who invented an explosive, was called "The Merchant of Death" in his lifetime, but is now remembered for his Peace Prize; Gates similarly does not wish to be remembered for the dirty and illegal tricks his company has played under his leadership; there were certainly those [there are some links in the Reference page] but they would be the subject of another website.