The case interviews represent a small snapshot of the daily work of a consultant. They evaluate several skills such as
- idea generation and creativity
- business sense and intuition
- mental math and affinity for numbers
- logical reasoning
- analytical rigor
In theory, if you perform well in the case interviews, you should be able to perform well on the job. For McKinsey, there is a high correlation of candidates passing the interviews (case + PEI) and performing well on the job up until EM level so it is definitely an instrument that works well and is battle-tested over decades of interviewing and hiring.
The demands put on employees once they join are several magnitudes bigger than what you have to do in the case interview though, which is also the reason why MBB candidates are sought after highly in the job market once they leave consulting. Few people are able AND willing to do it (MBB are often actually struggling to meet their hiring targets).
In that sense, the cases are just the beginning and if you don't enjoy doing them you might have a hard time on the actual job.