It is provided for those sites that may not have a Fortran 77 compiler available. For example, the current Linix version was built from the C source code.
Performance is comparable to the Fortran version.
We do not maintain a separate C version. Instead, all updates are done on the Fortran source and a new version of the C code is generated by running the f2c program again. If you intend to make local source changes, we recommend that you do the same. The f2c does a fine job of generating efficient C code. However, as an automatic code generator for a rather large source program, it does tend to generate rather unreadable C code.