If your kit was manufactured before 1995, you will probably need the sprocket and idler. After that, only if it's not from one of the "Big Three". The only set I ever used was back in the early-mid 90s for the Tamiya Pz IVD because that kit has 40cm sprocket/idlers rather than 38cm. I used the Fruil s/i with Kasten tracks which was very high-tech back in 1994 or so.
Since most of those Fruil sprockets and idlers were made a decade or more ago, the injection-molded ones are probably as good or much better today.