When you deploy both a JAD and JAR, normally, you would send a WAP Push
service indication message to the phone where the URL points to the JAD
file.

When you open the WAP push message and click on the URL, phone recognizes
that the file being downloaded is a JAD file, and the J2ME on the phone
then displays the information specified on the JAD file to the user prior
to downloading the JAR, which is also on the JAD file. If you respond with
yes, then the phone downloads the JAR file and installs the app on your
phone.

Of course, you can tell the phone to download the JAR directly and byass the
JAD, but the purpose of the JAD is really as a confirmation to the user
that they want to install the said application from the said vendor, whose
size is x Kb, etc..