When you select "Update Profile", your device connects to sprints data network and makes a request to the OMADM (open mobile alliance device manager), which in turn pushes a provXML file to your phone with your MDN, MSID, username, primary and secondary HA ip addresses, HA password and AAA password. There are probably a handful of other values involving wimax and such but I don't know what they are.
This information is what your phone uses to authenticate itself on sprints network for voice and data. If any of these values were to be incorrect then you could have issues make/receiving calls and/or connecting to data.
"Update PRL" makes the same type of request to the OMADM, in this case though it sends your Preferred Roaming List. This is basically a list of tower Ids and capabilities. Carriers will often release new PRLs as towers get built/updated and roaming contracts with other carriers change. It saves them lots of money to be sure you are roaming on the cheapest tower since they have to pay the respective company for your airtime.
This was probably not the answer you were looking for but... I was bored and thought I would share since I didn't like what Google spat out.