Can I explain in English? I can write English much faster.
Well, 'zum' is correct. It is short for 'zu dem'. In principle you could leave out 'dem' and the sentence would still be understandable. Like in the English 'They use the flash light to read'. (That is what you were thinking of right?)
But German uses grammatical cases. It's extra information. Dem is expressing the grammatical case Dativ. It says, in this case, you are using the flash light as a way to make the reading possible. It's your instrument or tool. So basically the poster getting the -1 above me, is totally right. If, in German, you would not specify 'dem'=='dativ'== 'instrument', it could mean you are trying to read something written on the lamp.
German and English come from the same Germanic language root. English has thrown away these grammatical extra info parts, because mostly it is clear what is the 'instrument' and what is the 'goal'. But German still uses the extra info. Germans are more precise. It's their nature :-)
By the way, in the grammatical cases there are far more things than what I just explained. There is Genetiv, Dativ, Akkusativ. And for 'the' in English they use 'der', 'die', 'das', 'dem', 'den'. And there are more rules and reason to use one or the other. But that goes beyond the scope of just this question.