I quote from page 30 of getting started with uvision2 keil compiler user's guid:
The C51 Compiler generates object code that matches the efficiency and speed of assembly programming

using high-level language like C has many advantages over assembly language programming:

* knowledge of the processor instruction set is not required. Rudimentary knowledge of the memory structure of the 8051CPU is desirable(but not necessary).

* Details like register allocation and addressing of the various memory types and data types is managed by the compiler.

* program gets a formal structure(which is imposed by the C programming language) and can be devided into separate functions. This contributes to source code reusability as well as better overall application structure.

* The ability to combine variable selection with specific operations improves program readability.

* Keywords and operational functions that more nearly resemble the human thought process may be used.

* Programming and program test time is drastically reduced.

* The C run-time library contains many standard routines such as : formated output, numeric conversions, and floating point arithmetic.

* Existing program parts can be more easily included into new programs because of modular program construction techniques.

* The C language is a very portable language(based on the ANSI standard) that enjoys wide popular support and is easily obtained for most systems. Existing program investments can be quickly adapted to other processors as needed.