Renaming types without changing their exposed semantics/characteristics doesn’t make much sense. In your example
typedef unsigned char uch;
typedef unsigned long ulg;
belong to that category. I don’t see the point, aside from making a shorter name.
But these ones
typedef uch UBYTE;
typedef unsigned int u32;
typedef signed short s16;
are a completely different story. For example, s16 stands for “signed 16 bit type”. This type is not necessarily signed short. Which specific type will hide behind s16 is platform-dependent. Programmers introduce this extra level of naming indirection to simplify the support for multiple platforms. If on some other platform signed 16 bit type happens to be signed int, the programmer will only have to change one typedef definition. UBYTE apparently stands for an unsigned machine byte type, which is not necessarily unsigned char.
It’s worth noting that the C99 specification already provides a standard nomenclature for integral types of specific width, like int16_t, uint32_t and so on. It probably makes more sense to stick with this standard naming convention on platforms that don’t support C99.