To prevent air leaks where the diaphragm contacts the air chamber. To prevent a diaphragm from pulling out of place if there is excessive air pressure.
On the thermoplastic diaphragms (TPO, TPE, PTFE), there is no o-ring molded in the diaphragm. Instead a separate o-ring is used to fill the o’ring groove in the air chamber wall.
The new style diaphragms cannot be used in the old style pumps, however the old style diaphragms can be used in the new style pump when used with the air chamber wall o-ring. The old style diaphragm will continue to be available for a few more years.