The ancestors of the x86 are the 4004 (4 bits) and the 8080 (8 bits). Devices of these ancient times often coded their instructions in octal since bytes were in short supply, they had to be used as efficiently as possible. This is the legacy we have today. The addressing byte consists of

Simplistically:

mod: 2 bits addressing mode
reg: 3 bits register1 (normally the destination)
r/m: 3 bits register2 (normally the source)

when mod=00 register2 hold the memory address
when mod=01 register2 holds the memory address + offset (next byte)
when mod=10 register2 holds the memory address + offset (next 4 bytes)
when mod=11 register2 is treated as the location of the data (reg to reg transfer)

Example: mov 32 bit
8b c1 ' mov eax,ecx ' going from right to left in Intel syntax
8b c8 ' mov ecx,eax
8b 01 ' mov eax, [ecx]
8b 41 08 ' mov eax [ecx+8]
8b 81 00 02 00 00 ' mov eax,[ecx+512]

But the x86 has elaborated on this scheme to extend the addressing modes (like irregular verbs in natural languages), so for some combinations its a bit more complicated.