First of all, let's be clear on "styles" and discount the low-mid grade black belt who decides to become the master of his own "style." He may invent his own kata to give credence his uniquely created style, and/or because he doesn't understand or like the ones he was taught. This is often a mishmash of whatever they guy thinks looks good or reflects his genius in reinventing the wheel.
If we're talking about traditional styles that have remained true to their roots, there are kata that are 100-250 years old, and yes, they've been modified over the generations but usually still recognizable. These forms reflect the real combat methods fighting professionals used including vital point strikes, limb grabs, twists and breaks, takedowns, etc. (Isshinryu founder Shimabuku did create a "new" kata, sunsu, but this is just a combination of techniques from several other kata, making it the style's advanced "master form," reflecting the style's total identity, technique wise).
As karate spread to the Japanese public schools, most of these more lethal techniques were "removed" from these traditional forms (I put "removed" in quotes as they could be said to still exist, but not overtly taught or shown, sometimes to the extent they were forgotten.)
Some traditional styles did create their own new kata for beginners not wanting to take a year to learn and understand the old more complex ones, and for those that didn't, individual teachers would create their own beginning "dojo kata."
Lastly, there are newer modern styles that have evolved with their own kata. These usually stress competitive techniques or are a collection of selected techniques taken from other systems to give an MMA flavor to it.