The value of a cryptocurrency changes over time due to the scarcity of the cryptocurrency. Cryptocurrencies are a resource and they are a limited resource. The price of a resource is the cost of acquiring it from the market. This price is affected by the scarcity of the resource. Scarcity is the gulf between a limited supply and a potentially limitless demand. There are criteria of acquisition for any resource. For example, the criteria of acquisition of a normal stone are simply to walk outside and pick one up. These criteria are easy to satisfy. However the criteria of acquisition for gold are far harder to satisfy: to acquire gold you must either set up huge mining operations or pay the price of gold to buy it off the market. Why are the criteria of acquisition for gold so much harder to satisfy than the criteria of acquisition for a normal stone? This is because gold is more scarce than normal stones. The more scarce a resource is, the harder it is to satisfy the criteria of acquisition. This also means that if scarcity is introduced to a resource, the criteria become harder to satisfy. When it comes to the coins of a cryptocurrency, the criteria of acquisition are mining/staking or paying the price of the coin and buying it from an exchange. These are the criteria that will be affected by the scarcity of the cryptocurrency.
There have to be limits on the total supply of a resource for it to have scarcity. This much is pretty obvious: in day to day life, access to the air we need to breathe is easy and the total supply is effectively limitless. No one charges us to breathe air because no one would pay for it. It is vitally important that a value-bearing currency has a limited supply. Fiat currencies achieve this by having the central authority control currency creation and then enforce regulation with a strict legal system. There’s a reason why being caught with a counterfeit US dollar can come with a 20 year jail sentence, even if the person didn’t counterfeit it themselves. Cryptocurrencies similarly need to limit total supply of coins and control coin creation, but they do it through the code itself.
Scarcity can be demand-induced or supply-induced. Demand-induced scarcity occurs when the overall demand for coin is increased, but the total supply remains the same. This could be because the population wanting the coin grows or because the individual demand for the coin grows. Supply-induced scarcity occurs when overall supply is reduced. For example, some cryptocurrencies burn their transaction fees rather than give them to the miners or stakers, which means the total supply of the coin goes down when it is used. To understand how the scarcity of a coin might change over time, we need to have an understanding of what drives demand for the coin. You can read more about how the utility of a cryptocurrency creates demand for the coin in the answer to the question, What key features of a cryptocurrency induce scarcity and hence affect the price of the cryptocurrency?