As new and interesting technologies arise Microsoft tries to immediately imitate it and somehow bundle it in its Windows OS. Microsoft now has 'tailgated' on the success of JavaScript. JavaScript has been the scripting language of choice for the client side web development since its early development at Netscape. When Netscape and Internet Explorer where battling out the Browser War, the JavaScript implementation used in each browser diverged. To do something interesting and cross browser compliant you had to have code that basically said "if this browser is IE do it this way otherwise do it the normal way."
On the sever side, Perl had gained a large acceptance especially by System Administrators. Microsoft quickly tailgated the success of Perl and set out to develop its Windows Scripting Host which can interpret script based languages. To run on top of the Windows Scripting Host, Microsoft developed JScript. The one feature that I like about JScript is that you can easily compile it to a windows native exe. But this feature is what makes this language suck. I mean, I also use Linux and Mac X OS and want to program in a cross platform environment. But when working in an exclusive Windows environment, this scripting language might come in handy.
There are a few other benefits for the Windows only users. Since JScript runs on the Windows Script Host it can be used as a script language in any application that supports Windows Script. Again, these are benefits is you heavily depend and use Windows and other Microsoft products.
Overall, JScript itself has enough nuance and difference between C, C++, Java, JavaScript, and Perl that if it is not necessary for you use this language stay as far away as possible. The Windows Host also supports other script languages including Visual Basic and Perl.