Is it necessary for preschoolers to have computer skills?

No, it is definitely not necessary for preschoolers to have computer skills. Children can learn the same skills in many other ways (skills such as eye-hand coordination and familiarity with numbers and letters). Preschoolers will certainly become familiar with computers and computer skills as they enter the public school system. However, most children enjoy the computer, and it can be another way for them to explore and learn. Most of the new computer programs are quite open-ended, providing opportunities for problem solving and thinking skills, rather than just rote quizzes (as they were when they first came out). Most schools will have opportunities for preschoolers to experience computers, if you do not have one at home.
In summary, a preschooler's learning will not be limited by not having computer experience, but computers can be a fun and challenging way to enhance learning if they are available.