The SPIM documentation reads:
SPIM S20 is a simulator that runs programs for the MIPS R2000/R3000 RISC computers. SPIM can read and immediately execute files containing assembly language. SPIM is a self-contained system for running these programs and contains a debugger and interface to a few operating system services.
The architecture of the MIPS computers is simple and regular, which makes it easy to learn and understand. The processor contains 32 general-purpose 32-bit registers and a well-designed instruction set that make it a propitious target for generating code in a compiler.
However, few years ago, the obvious question was: why use a simulator when many people have workstations that contain a hardware, and hence significantly faster, implementation of this computer? One reason was that these workstations are not generally available. Another reason was that these machine will not persist for many years because of the rapid progress leading to new and faster computers. Unfortunately, the trend is to make computers faster by executing several instructions concurrently, which makes their architecture more difficult to understand and program. The MIPS architecture may be the epitome of a simple, clean RISC machine. Nowadays, the MIPS architecture is no more a common architecture.
In addition, simulators can provide a better environment for low-level programming than an actual machine because they can detect more errors and provide more features than an actual computer. For example, SPIM has a X-window interface that is better than most debuggers for the actual machines.
Finally, simulators are an useful tool for studying computers and the programs that run on them. Because they are implemented in software, not silicon, they can be easily modified to add new instructions, build new systems such as multiprocessors, or simply to collect data.