STEP does show an interest in maths beyond the curriculum, and aside from helping with applications, it will also help you prepare for university-level maths.
If your school is willing to pay for it and you're willing to put in the effort to prepare properly, go for it; it won't hurt your chances, although it also won't increase them massively. Do bear in mind, however, that the time taken to properly prepare for STEP is quite substantial. How much time you should spend is a whole debate in itself, but I've heard you should spend approximately as much time as you would on a whole a-level; I started in January doing about an hour a day, stepped this up to 2 hours ish a day in April, and was doing between 8 and 10 hours a day for the 4 weeks before the exams. There's not much point in doing it half-heartedly, so be aware of the commitment you're making.
Which universities do you want to apply to? Do they ever give offers involving STEP?