$BTC

Each block can only contain a certain number of transactions, and that number is determined largely by the space available in each block, or the 'block size,' which is 1MB. The limited space gives rise to the fee market, where miners, who collect fees, choose to include in the next block only those transactions which have included a high enough fee. Thus higher fees act as incentive for miners to prioritize your transactions.

Note that the block size is an arbitrary limit, but the Bitcoin community has chosen to keep the block size as small as possible in order to make it easier for people to operate Bitcoin nodes. Bitcoin Cash, which is a fork of Bitcoin, has a larger block size and therefore requires much lower fees for transactions.