That doesn't mean that it was used primarily with C++ though. IIRC Watcom C/C++ mainly became popular because of Doom, and that was written in C (as all id games until Doom 3 in 2004 - again IIRC though).
The actual killer feature of Watcom C/C++ was not the C or C++ compiler, but its integration with DOS4GW.
Borland C++, Microsoft C/C++, and GCC (DJGPP[1]) could all target 32-bit extended DOS, but Watcom was the first[2] to bundle a royalty-free DOS extender[3].
The actual killer feature of Watcom C/C++ was not the C or C++ compiler, but its integration with DOS4GW.