people could actually buy collections for the sake of collecting, perhaps out of admiration, but there are those that buy for the sake of saving their money through collections, you see them buy many of these collections stack them for the future rather than having those money sitting in bank accounts where eyes could easily notice their networth. This is just how some of these wealthy people could choose to store their wealth against the future and they can always sell any whenever they have a demand for bulksum of money and the chances are that the could sell at a price 2x of what they bought it.
There is no certainty that the price of this artwork will appreciate in the future. Sina Estavi bought Twitter Jack Dorsey's first tweet as an NFT at the price of $2.9 million because he thought the price would skyrocket in the future. When he wanted to sell it the highest bidder priced it for $277. This example doesn't dispute the fact that other NFTs' value have increased.
Some NFT analysts believe that people buy these NFTs artwork to build brands around them. They gave an
example of the Bored Ape. This NFT artwork has been used by many companies to brand products. Since this artwork is popular business products can be branded and promoted with these NTFs.