Why is there a need for a Top Altitude in FAA's SIDs?
It is needed for the new climb-via instruction (c. 2014).

Instead of issuing different maintain altitudes to the aircraft based on the SID, direction, etc., a top altitude is charted. From the FAA AIM:


  SIDs will have a "top altitude;" the "top altitude" is the charted "maintain" altitude contained in the procedure description or assigned by ATC.


Related:


Do Jeppesen SIDs have top altitude?
What altitude should you cross DBIGE at on the PALLEY FIVE Departure (SID)?
Are &#39;climb via a SID&#39; instructions actually used in the US?