Hi Romani,
Welcome to the forums!
The key to understand the necessity of Header Length is to realize that with IPv4 the size of the header is not fixed (like it is in IPv6). The size of the IPv4 header must be at least 20 bytes, but it can be bigger, too. What makes it bigger are the additions of “options.” To learn more about options go here
Since the size of the IPv4 header is variable, the purpose of the Header Length is to specify just how big it actually is, but there are rules as to what sizes are allowed. As mentioned earlier, the minimum is 20 bytes, and the maximum is 60 bytes, but in-between those values, the size is only allowed to increase in increments of 4 bytes (since 8 bits = 1 byte, 32 bits = 4 bytes). So, for example, the possible sizes (in bytes) are 20, 24, 28, 32, … 60.
The actual value of the “Header Length” field is a binary number that represents how many sets of 4 byte “blocks” there are (or how many sets of 32 bit “blocks” there are–its the same thing). Remember, we said a minimum value was 20 bytes? How many “4 byte blocks” are there in 20 bytes? 5, right? So 5 would be the minimum value for Header Length. In the case of the maximum header length, it would be 15.
Finally, remember that actual value in the Header Length is a binary number–specifically with 4 bits. That means the largest possible value is “1 1 1 1” which in decimal = 15. This is also the limit of the maximum header size–60 bytes.