When should we use this attribute and why do we need it? For example, if the native function in c takes as a parameter a pointer to unsigned char, and I know that it's needed to fulfill the array of unsigned chars, why can't I use array of bytes in C# to use this function? Is it necessary to do marshalling?
The runtime will be able to automatically determine how to marshal data between native and managed code in most cases, so you generally don't need to specify the attribute. MarshalAs is only necessary when there is an ambiguity in the definition (and you want to tell the runtime precisely how to marshal the data) or if you require non-default behaviour.
In my experience, MarshalAs is only really required when working with strings, since there are so many different representations in native code; unicode/ansi, c-strings or not, etc.