The short answer is they are very important.
Good experienced jockeys are top class athletes who are skilled at communicating with the horses carrying them around the track at breakneck speeds with only their hands, a couple verbal cues and the “whip” – actually, a cushioned crop designed to produce maximum noise without raising welts or otherwise causing injury.
Handicappers must, to some extent, follow the jockeys as the top riders on a given circuit shift from one horse to another in the constant pursuit of the next win. That doesn’t mean you blindly bet on them, but you need to at least consider why they took off one mount in favor of another.
When a top jockey is replaced by a journeyman rider with inferior statistics or an apprentice rider, it’s reasonable to project that there will be a drop-off in performance. That doesn’t always turn out to be the case, but the top riders are amazingly adept at coaxing top performances from all kinds of horses.
Also, keep an eye out for horses that seem to respond to a certain jockey. We don’t know what goes on in their minds, but some horses just seem to do their best for a certain rider who they instinctively trust.
Finally, get familiar with the riders on the circuit you play. Are there specialists who do especially well with sprinters or turf runners?