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How the Outer Beings Can Touch the Prime Planeby Geoff Gander
The way I imagine things, in the old days, the OBs operated pretty much like Immortals - they granted spells, manifested occasionally on the Prime Plane (they didn't really favour avatars - worshippers got the real deal), and pretty much did what they wanted. After they were sealed away, they were generally unable to directly affect the Prime Plane, but (as with Lovecraft) they may still reach certain people in their dreams, or when they attain a mental state in which they could be more open to contact (i.e., through the use of drugs, intense meditation, magic, acquiring knowledge, etc.)
In addition, the barriers that prevent the OBs from touching much of the multiverse fluctuate over time. At certain times of the year they are slightly weaker, and at those times people can be more susceptible to being influenced. Certain elements of the barriers may also tied to the stars themselves, and as they move through the heavens, the barriers likewise strengthen and weaken. Thus, there may well come a time when "the stars are right", and the barriers will be weakened enough to allow the OBs to break through, if the right rituals are performed.
Some of the tomes created for use in campaigns would have been written after some particularly bad dreams, or (again in admiring imitation of HPL's style) after the writer "learned too much". The in-game mechanism for this is the Outer Being Lore skill. As a person learns more, they attract the attention of the OBs because their mind becomes progressively more attuned to them. Thus, even though the OBs are locked away in other dimensions, the scholar (voluntary or not) of OB lore stands out like a beacon. Once first contact is made, the link becomes stronger over time.
Now, in the case of cultists, who willingly open their minds to the OBs, this process is greatly accelerated. This process is often accompanied by a specific ritual; it would be insufficient for a person to simply decide to start venerating the OBs. Cultists with high Wisdom may even be granted spells*; although to a lesser degree than would be the case with conventional clerics, whose Immortal patrons are not inhibited from touching or otherwise influencing the Prime Plane.
In addition to all this, the OBs' servitors play a role, as you rightly point out. Being closely aligned with the OBs, and most often being *of* them, servitors can often act as emissaries and catalysts, amplifying the strength of prayers, and allowing the OBs to occasionally push through the barriers to grant "gifts" to the faithful.
*Someday soon, I may get around to writing up an OB cultist NPC class, much as was done for the anti-paladin. It might be useful to DMs as an aid for creating cultists for the PCs to fight.