Hexadecimal numbers are more readable and compact than binary strings and thus easier for human beings to work with. So for example when a programmer creates or maintains a configuration file with the colors for a website, or a constants file with numeric constants, working with hexadecimal numbers is easier than working with long binary strings.
For example, FF0000 (red) is more compact and readable than 11111111,
00000000, 00000000 which is red in binary.
Now decimal could also be used here instead of hexadecimal, but it is less space efficient than hexadecimal because a hexadecimal string can represent a much bigger range of numbers than a decimal string of the same length.
For example, FFFF, the largest hexadecimal number of 4 digits is equal
to the decimal number 65535 which is far bigger than 9999, the largest
decimal number of 4 digits.
Also, of course Hexadecimal digits (0-9, A-F) use all 16 possible combinations of a 4 bit binary string while Binary digits (0-9) use 10 of the 16 combinations.