Just the peace of mind that your application was received and some movement has started on the application.
Also what the IRCC lady mentioned was that the temp number starts with a XF(9digits) and once the application is checked for completeness the X will be removed and the rest of the number would become your Application number. Thats what she said, not sure if thats the case with all the applications. Below is how she explained it to me. Hope that's helpful!
X(indicates its temporary)
F(stands for family)