No one uses this expression anymore because it is a very oldfashioned one.

A mostly British expression urging someone to stick to the thing he knows best, 'horses for courses' comes from the horse racing world, where it is widely assumed that some horses race better on certain courses than on others. A horse that runs well on a dry course will run less well on a damp course and vice versa." it means that what is suitable for one person or situation might be unsuitable for another.

Horses for courses. (British & Australian) something that you say which means that it is important to choose suitable people for particular activities because everyone has different skills Ah well, horses for courses. Just because a plumber can mend your washing machine, it doesn't follow that he can mend your car as well. www.thefreedictionary.com

Horses for courses===Choose best people for a job.

"Courses" here means "race courses" or "racetracks". The image is that some horses might right a particular race track better than others.