Critical discussion: Gene expression and systems biology allows us to understand the control of complex traits
Turns to be too long for a comment.

If you are looking for explanation of the title/question, than it is following (roughly).

If we get information about gene expression from all different cell types, and extract also interactions between those genes (if a gets higher then b gets lower), this will allow studying multigene traits. E.g., life quality, height, mental disabilities, cardiovascular risks etc. Thing is that for most health problems that stem from single gene variation we already know named gene, or can easily find it out.

More complex issues are often hidden in statistics, links are not so clear. So, for example, autism is described by genetic variations (so far) only in less than 30% of cases or so. In other cases there is no genetic predisposition or strong evidence. Mainly that is so because many genes interact to produce final trait or illness. Looking with microscope one gene at a time will not provide information about it.

You should look into multi-gene traits, maybe pick your favourite illness (e.g. autism) and see how it is hereditary (tip: not fully). Whole-genome sequencing is important tool in tracing such issues because it gives wide overview and a lot of genetic information about patient.