They are an very important part of the process of development of treatments and for the understanding of how certain cells or substance-cell interactions work. However what an in vitro study does not and will never do is to give proof of something working in a living organism. And a lot of comments here seem to assume this study proves the efficacy of CBD in the treatment of cancer which is plain wrong. Asking yourself "is this an in vitro or an in vivo study?" will make you less likely to come to a wrong conclusion about the significance of this particular study.