Tomcat enables some additional debugging options at start up if you are running with a full JDK. These options require the JDK so you Tomcat checks you are actually using one if you claim that you are to ensure these options don’t fail if used.
I rarely see these options being used. I think I have used them once in getting on for 10 years working with Tomcat.
When you use JRE_HOME Tomcat doesn’t enable JDK specific options so it doesn’t check if you are running with the full JDK rather than the JRE.