Understanding serialization

I am a software engineer and after a discussion with some colleagues, I realized I do not have a good grasp of the concept serialization. As I understand, serialization is the process of converting some entity, such as an object in OOP, to a sequence of bytes, so that the said entity can be stored or transmitted for subsequent access (the process of "deserialization").

The trouble I have is: aren't all variables (be it primitives like int or composite objects) already represented by a sequence of bytes? (Of course they are, because they are stored in registers, memory, disk, etc.)

So what makes serialization such a deep topic? To serialize a variable, can't we just take these bytes in memory, and write them to a file? What intricacies have I missed?

If you have a complicated data structure, its representation in memory might ordinarily be scattered throughout memory. (Think of a binary tree, for instance.)

In contrast, when you want to write it to disk, you probably want to have a representation as a (hopefully short) sequence of contiguous bytes. That's what serialization does for you.

StackExchange Computer Science Q#72102, answer score: 42