When you receive a certain amount of All Skills and Powers experience, you receive that amount of Specific experience to all of the skills and powers in the game. Thus, the All Skills and Powers experience is a very valuable asset. For veteran characters who have all the 18 available skills and 11 powers, plus a Quickstone, 100 AS&P XP is equivalent to 3190 General XP.
All XP to Skills and Powers you do not yet possess is now stored in A windowless building with a blue door in Hawklor, Talinus and Trithik. You can acquire all the XP for newly newly received skills and powers there at any time. AS&P XP was not stored before 11/25/2015, causing many players to tend to avoid quests with high AS&P awards until they had most or all of the available skills and powers. Nowadays there are no gameplay reasons to delay doing these quests after their MR and skill/power usage requirements have been reached.
Many players tend to use only Specific XP, most of it gained through successive AS&P XP awards, to level up their skills and powers, saving their General XP for further leveling one ability in case it's needed later.
See the list of AS&P quests.