"Why do some English words have a double-consonant?" Well, the short answer is that they do and you need to learn it that way. Apart from that, sometimes the double-consonant exists to make the vowel before it short. Compare "tiny" (long i) and "tinny" (short i). Some short verbs need a double consonant when you add -ing, eg. "stop / stopping". If you write "stoping", then the o is long... and it's not even a word that we recognise. There are many conflicting rules for English spelling, so if you rely only on phonetic rules you will make many mistakes. Just accept the spelling, remember it and you'll have no problem.

Contrary to popular belief, the English language does have rules that descend from the Anglo-Saxon era. Double consonants in English affect pronunciation and sound(stress) Spell without the double "LL" would be pronounced more like spill. "Hammering" double consonant needed to indicate that "m" is pronounced twice -these are just a few examples-

Honestly the best answer is, "English is stupid language at times". English is at its best an amazing amalgam of languages from all over Europe and other countries across the world. We have literally borrowed rules and words from everywhere across a whole lot of history. The danger is that the sounds the double vowels can make change depending on the consonant following it. For example: Loot, Loose and Goose sound different to Foot or Soot and different again to Wool or Fool or for that matter. If in doubt about why something is spelled a particular way, just realize that English is a dumb language at time even to those of us who were born in an English speaking home.

Like Peachey said, double letters are often used to differentiate long and short vowels. You should be aware that there are lot of exceptions in English spelling, partly because words were introduced into English from so many different languages and partly because English spelling hasn't been updated for a long time, so the way words are spelled often reflect ancient pronunciations.