Why do we use hexadecimal in computers instead of just binary?

Hexadecimal exists to make long binary numbers shorter and easier for humans to read and write, without losing the connection to binary. Because 16 is 2 to the power 4, exactly one hex digit represents four binary bits. So an 8-bit byte like 11111111 becomes just FF, and the 4-bit groups map cleanly: 1010 is A, 1111 is F. This is why memory addresses, colour codes like #FF8800, and error codes use hex, since it is compact and converts to binary instantly. Computers still work in binary internally; hexadecimal is only a convenient shorthand for us. Octal works similarly with groups of three bits, but hex is preferred because modern data is organised in bytes of eight bits, which split neatly into two hex digits.