This boost is the best Farming boost in game, providing a consistent +5 boost regardless of level. (The Stranger plant can only boost a maximum of +4 even at Level 99 Farming.) However, the item is quite hard to obtain over the Grand Exchange, so making one yourself is a viable option.
The pie also boosts your summoning points. Although you gain a Summoning boost of +5 which is visible, this boost is merely an increase in Summoning points, and will not allow you to perform any tasks requiring a higher summoning level than what you have initially, other than opening Summoning-affiliated ancient effigies. The summoning point boost can be useful when renewing a familiar while fighting or skilling for a long time in a location far removed from any mini-obelisks.