In addition to what ILuv said, there are two more reasons for requiring research experience.
1) PhD’s and academia in general are very research intensive. Having research experience lets them know that you understand what you are getting yourself into.
2) Experience in a lab suggests that you will be familiar with processes, programs and protocols, and therefore, easier to train so that you become an asset to your PI in a shorter time period.
Research experience means different things in different contexts, but keeping the previous two points in mind should help you recognize “good” research experiences.