If the forward ends of the nacelles were not protected by the main sensor-deflector (like other parts of the ship) they'd be exposed to hazardous impact of stellar debris.
Or, in other words, in the best possible position to sweep the ship's path clear of this debris.
Whether by suction alone (with the debris matter stored somewhere for later use or dumping, or incinerated), or by combination of suction and subsequent lateral venting, we don't know. But if the domes suck, the rings aft of them could blow. Or vice versa, as we only ever see the domes blowing, in two TNG episodes, before the nacelle style changes to domes-only and we see them suck in ST:INS.
Although in this pair of images, it does seem as if the "domes" undeniably suck yet an unidentified feature aft of them, perhaps along their aft edge, might be responsible for the blowing.