It seems that you have not had the opportunity to study basic digital processors which are at the heart of computers and PLCs. They store numbers in groups of bits. The most efficient storage of numbers is using a base which is a power of two. Groups of bits which have been used are 4, 8. 16, 32 and 64. None of these store the value 10 as a basic unit. Thus you see binary (single bits used one at a time) and hexadecimal (bits used 4 at a time). These are used for the most efficient and fastest processing. Some PLCs have BCD (binary coded decimal - 4 bit groups limited to the values 0 - 9) available to enable compatibility with much older applications. This is at a cost of much slower processing and decreased efficiency.